The praise of Allah in this pillar is to bow his back down, lowering his head while his tongue utters the words: Subhana Rabbiyal Adhim, “Glory be to my Lord, the Greatest”. Through this act, his heart, body and tongue submit to Allah in a most sublime manner that brings together the feeling of humility, humbleness, Allah’s glorification, and His remembrance – thus uniquely distinguishing his submission to Allah from the submission of the slaves (i.e. people) to one another. This is because submission is the attribute of slaves and Glory is the attribute of the Lord.
The perfect state of servitude that manifests while bowing down is when the slave dismisses his ego, accepts his status as a slave of Allah, and empties his heart from any glorification he might have had about anyone besides Allah and replaces all that with is glorification of Allah as the only Ilah worthy of worship.
When the heart is overwhelmed by his glorification of his Lord, his glorification of himself or of any other created being will be dismissed from is heart.
Not withstanding the outward physical movement of the body when bowing down, this pillar is in reality an act of worship that is foremost prescribed to be performed by the heart; the physical movement of the body is a mere accessory to complete the pillar.
Inner Dimensions of the Prayer by Ibn Qayyim Al-Jawziyyah Page 48