Know that, as the adhan (call to the Salat) is the announcement of the presence [hudur] of the external and internal powers of the soul in the Presence [mahdar] of Allah for the sake of praising His Sacred Essence according to all the Names, Attributes, Affairs and ayahs -since the Salat, as has already been mentioned, is a comprehensive praising of the Sacred Essence according to the manifestation of the Greatest Name, which is the state of "the Collective Oneness of the Names" [ahadiyat-i jam'-i asma'] in His Unity [hadrat-i wahdiyat], and the state of manifestation through the collectivity [jam] differentiation [tafriq], overtness [zuhur] and covertness [butun] in the essences [ayan] and essential names [asma-i ainiyah] -the salik's attention is first directed to the Majesty of the Sacred Essence according to this general affair [sha'n-i jami]. So, at first, he introduces the said Greatness and Majesty to the invisible and visible powers of his own kingdom. Then, secondly, to the angels of Allah in charge of the invisible powers spread in the kingdom of the soul.
Thirdly, to the beings of the invisible and visible worlds, and fourthly, to the angels of Allah in charge of the Kingdom of the heavens and the earths (lands = aradin). So, through these four takbirs (saying: "Allah is Greater") he announces the Majesty of the Grand Name (Greatest Name) to all the dwellers of the invisible and visible worlds of the inward and outward kingdoms. And this, by itself, is an announcement of his being incapable of undertaking the duty of praising the Sacred Essence, and an announcement of his falling short of performing the salat. This, in itself, is one of the general affairs of the suluk and of the comprehensive disciplines concerning praising and worshipping, which must be before the salik's eyes during the whole period of performing the salat. That is why the takbir is repeated in the adhan and the iqamah, as well as in the salat. It is also repeated when passing from one stage to another so that the salik's innate inability, and the Greatness and the Glory of the Sacred Essence are confirmed in his heart.
As such, its discipline, as it appears, is that the salik should, in each takbir, remind himself of his inability and Allah's Majesty. On another supposition, it is possible that each one of these preliminary takbirs of the adhan points to a state: The first takbir means: He is Greater than the attributive Essence; the second means: He is Greater than the attributive Attribute; the third means: He is Greater than the attributive Name; and the fourth means: He is Greater than the attributive Act. Thus, it is as if the salik says: Allahu akbar [Allah is Greater] than your description of His Essence, or of His Essence-manifestations, and He is Greater than your describing Attributes, His Names and His Acts, or their respective manifestations.
In an elaborated speech quoted from Amirul Mu'minin 'Ali (AS) it is said: "... The other aspect is that Allahu akbar implies negation of His quality, as if he (the mu'adhdhin = the one who speaks out the adhan) says: ' Allah's attribute, with which He is qualified, is far above being comprehended by the describers', for the describers describe Him according to their own measure, not according to the measure of His Greatness and Majesty. He is far above His quality being understood by the describers ..." as the hadith goes.
Another important discipline of the takbirs is that the salik is to strive, and, by cordial austerities, he is to prepare his heart to be the place for the Majesty of Allah, the Glorified, and to regard Greatness, Glory, Sovereignty and Majesty to be exclusively ascribed to the Sacred Essence of Allah, the Most High, and to exclude the others from Majesty. If he feels in his heart even a tiny bit of anyone else's greatness, without taking it to be the light of that of Allah's, his heart is sick and is controlled by Satan. It is quite possible that Satan's intrusion would cause the sovereignty of the majesty of other than Allah, in the heart, to be more than that of Allah's, and the heart would regard him greater than Allah. In this case, man would be counted among the hypocrites. The symptom of this devastating disease is that man regards the pleasure of the creatures to be preferred to the pleasure of Allah, and in order to obtain the pleasure of the created, he would incur the displeasure of the Creator.
As-Sadiq (AS) is quoted to have said: "When you say: Allahu akbar, slight whatever is there between the high (heaven) and the earth, regarding it below His Majesty, because if Allah looked into the heart of the servant while telling the takbir, and saw therein something contradicting his takbir, He would say: " O you liar! Are you deceiving Me? By My Might and My Majesty, I will deprive you of (tasting) the sweetness of remembering Me, and I will exclude you from My proximity and from getting pleasure through your supplication. " 
My dear, the fact that our wretched hearts are deprived of the sweetness of remembering Allah, the Exalted, and that the enjoyment of supplication to the Sacred Essence is not tasted by our spirit, and that we are prevented from reaching the proximity of His Threshold and deprived of the manifestations of His Beauty and Majesty, is because our hearts are sick and faulty, attracted by the world, stuck to it and wrapped in the veils of the darkness of nature. And this fact deprives us of recognizing Allah's Majesty and of discerning the lights of His Beauty and Glory. As long as our look at the beings is Satanic and independent, we shall never drink of the wine of intimacy, not attain to the pleasure of supplication. As long as we believe that in the world of existence there can be glory, might, majesty, greatness and dignity for any created being, and as long as we are wrapped in the veils of the created specifications [ta 'ayyunat], the dominion [sultan] of the Majesty of Allah, the Glorified, will not manifest in our hearts.
So, of the disciplines of takbir is that the salik should not stop at its outer form, or be satisfied with its wordings and with mere pronunciations of the tongue. First, he is to prove to the heart, with the power of argument and the light of divine knowledge, Allah's Glory and the confinement of greatness and majesty only to the Sacred Essence of Allah, Most High, informing it of the poverty, humility and helplessness of all the possible dwellers and all the corporeal and spiritual beings. After that, with the power of austerity, frequent intimacy [murawadah] and complete familiarity, he is to enliven the heart with this divine grace and grant it spiritual and intellectual life and happiness. When the salik realizes the poverty and the humility of the possible (the creatures) and Allah's Greatness and Majesty, and puts that before his eyes, while his contemplation and remembrance reach their assigned limit, and the heart attains to familiarity and tranquillity, he will see with the eye of insight the effects of Allah's Glory and Majesty in all beings, and the diseases and faults of his heart will be cured. Only then will he taste the deliciousness of supplication and the sweetness of remembering Allah, and the heart will affirm Allah's Sovereign Majesty, and the effects of Majesty will appear in the exterior and in the interior of the kingdom, and the heart, the tongue, the outside and the inside will go in harmony. So, all the external and internal powers, visible and invisible, recite Allahu akbar, and one of the thick curtains is drown away, and he gets one stage nearer to the truth of the salat.
There is a reference to some of what has been said in a lengthy hadith Ilalush Shara'i, quoting Imam Ja'far as-Sadiq (AS) describing the Mi'raj. He said: "Allah, the Glorified and Almighty, sent down, to the Prophet, a carriage of light comprised of forty sorts of light which were around the' Arsh. The arsh of Allah, Blessed and Most High, blurs the eyes of the onlookers. One of them was yellow, and it became the cause of the yellowness of the yellow. Another one was red, and it became the cause of the redness of the red..." Then he added: "... He (the Messenger (SA)) sat in it and it ascended him to the lower heaven. The angels ran to the outskirts of the heaven, then they fell in prostration, and said: 'All-Glorified and All-Holy is our Lord, the Lord of the angels and the Spirit. How this light is like the light of our Lord!' Jibrael (Gabriel) said: :Allahu akbar! Allahu akbar!' The angels stopped talking, and the heaven was opened. The angels gathered and came to pay tribute to the Prophet (SA) group after group ..." as the hadith goes. 
In this noble hadith there are great secrets to which the hand of our hopes is too short to reach, and what can be said is now out of our purpose, like the secret of the descension of the carriage of light, the secret of the many lights, the secret of their diversity, the secret of the figure forty, the secret of its being sent down by Allah, the secret of their gathering around the 'Arsh, the truth of the 'Arsh in this respect, the secret of the yellowness of the yellow and the redness of the red caused by them, the secret of the angels' running, their bowing, praising and glorifying, and likening his light to Allah's, and the like. To speak about each of them would be lengthy. Yet, that which suits this occasion and testifies to our subject is that the angels of Allah quieted down as they heard Gabriel's takbir, and gathered around the candle of the meeting of the Absolute Guardian. By that takbir the first heaven opened, and one of the curtains, which blocked the way to Allah, was drawn away. It should be noted that the curtains which are pushed aside by the adhan are other than the curtains which are in the opening takbirs. We shall probably refer to this concept later on, insha 'allah (Allah willing).
Concerning there being only two takbirs in the iqamah, it is probably because the salik has set up his powers in the Presence, and has somewhat advanced from multiplicity toward unity, magnifying the Essence and the Names, or the Names and the Attributes; and it may be that the magnification of the Essence and the Names implies the magnification of the Attributes and the Acts.
. Biharul Anwar, vol. R1, p. 131, "Book of the Salat", ch. on "The Adhan and the Iqamah", hadith 24.
. Misbahush Shari'ah , ch. 13 , on " Finishing the Salat" . al Mahahhatul Bayda, printed by as-Saduq Library , vol. 1 , p. 385 , Mustadarak Wasa'il, 'Book of as-Salat", chs. on "The Acts of the Salat", ch. 2, hadith 9.
. 'Ilalush-Shara'i " vol. 2, p. 312, chs. on "The Causes of the Wudu" the Adhan and the salat", hadith 1.