Apparently, this takbir (saying: Allahu Akbar=Allah is the Greatest) belongs to the ruku ` (obeisance = genuflection) in order to be prepared for the stage of ruku `. Its relevant discipline is to bring to mind Allah's status of Greatness and Majesty and the Lord's Might and Sovereignty, and considering, at the same time, one's weakness, inability, poverty, humility and servitude. In such a condition, and according to one's knowledge of the Lord's Might and the servant's humility, one will say the takbir of the Haqq (Allah), the Exalted, being above all description.
The description [tausif] which the salik servant makes of the Haqq (Allah), the Exalted, and his glorifying and extolling, should be out of pure obedience, and taking them to be by the permission of the Haqq (Allah), the Exalted, to describe and worship Him. Otherwise he would dare not to vaunt of his describing and glorifying in the presence of the Lord, weak a servant as he is, and, actually, having nothing of his own, for whatsoever he has is from his Great Worshipped. When 'Ali ibn al Husayn, with such sweet and authoritative a tongue as he had, being the tongue of Allah, says: "How, with this feeble tongue as I have, can I thank you?",  what a thin gnat can do ?" 
So, when the salik servant wants to enter the grave stage of ruku`, he has to prepare himself for that stage, throwing behind, with his own hand, his describing, glorifying, worshipping and conduct [suluk]. He then has to raise his hands beside his ears, facing his empty palms towards the qiblah, and, empty-handed, with a heart full of fear and hope-fear of failure and deficiency in observing the state of servitude; and assured hope in the Holy state of Allah Who honoured him into such situations, which belong to the bosom friends of Allah and the perfect lovers - he enters the stage of ruku`. Probably, raising the hands in this way denotes leaving the qiyam (standing) position and the stand [wuquf] in that stage - a sign indicating taking no provision from the stage of qiyam. Takbir denotes glorifying and aggrandizing the descriptions done in the stage of qiyam. As ruku`, to the people of knowledge, is the stage of the Unity of Attributes, the takbir of ruku`, to them, is aggrandizing this Unity, and raising the hands is a sign of rejecting the attributes of the creatures.
. Misbahul Mutahajjid, p. 534, an extract from Abu Hamzah's invocation.
. Refer to footnote No. 272.