As the slave is about to bow down, it has been prescribed upon him to raise his hands as a show of his deference to the Command of Allah and to manifest the servitude of his hands to Allah, in adherence to the way of the Prophet, peace and blessings of Allah be upon him, and as a gesture that beautifies the salah. It is indeed the adornment of salah and the expression of one’s glorification of its parts.
Allah has then instructed that he pronounce the keyword of salah; that is to say takbir, which he must do every time he intends to move from one pillar (of salah) to another, as He has instructed the slogan of Hajj; that is to say the talbiyah [Labbayka Allah Labbayak – O Allah! We are obedient to Your orders, we respond to Your call] when the pilgrim moves from one act of the Hajj to another. All of this is so His slave knows that the secret of salah lies in extolling and glorifying Allah through worshipping Him alone.
The next act of bowing down is prescribed wherein the slave visibly demonstrates his submission before the Greatness of his Lord and shows humility before the Pride and Glory of his Lord.
Inner Dimensions of the Prayer by Ibn Qayyim Al Jawziyyah Page 47