After reciting the short chapter Fatiha in salat, you further recite a short chapter or a few verses from the Quran, then bow (ruku). Your upper body is parallel to the ground while your legs stand in a perpendicular position. 

While bowing you say “Allahu Akbar”.

When you were reciting the verses, you were in an upright position… The words of the Hayy and Qayyum One whose absolute authority encompasses the entire existence were being articulated through you… The domination of Allah, the solid divine system and order was being spoken through you, hence you were in a stable upright position. 

Then, since compositional beings effectively bow to the Rabb of the worlds, and your make-up is a composition of names, you bow to the Rabb of the worlds, allowing the requisites of the divine dominion to manifest through you.

Your lower body is upright, your upper body is parallel to the floor; with half your being you are fulfilling your servitude, and with the upper half, your consciousness and comprehension, you are bowing to the Absolute One, the Creator of the worlds; confirming His existence and Oneness…

Additionally…

The bent upper body that is parallel to the ground expresses one’s natural servitude. It is a depiction of recognition and submission to the Knowledge and Power of the Absolute One who is al-Hakim. This position shows that you have this cognition. 

On the other hand, with your lower body that is perpendicular to the floor you are depicting the perpetuation of existence with power of the Absolute One…

You are half upright, based on the Absolute Authority composing your compositional being, but half bent, based on the fact that your being is a composition which is dependent on the Absolute One.

Ruku, the act of bowing to Uluhiyyah, symbolizes the compositional beings made from the principles of Rububiyyah.

So then in this light, you declare:

“Subhana Rabbi al-Azim”

“My Rabb the Azim One, the magnificent glory beyond any capacity of comprehension, is Subhan!

It is His ruling and administration that is in effect in every iota of existence. Every single unit of existence is fulfilling its servitude by manifesting behavior aligned with its creational purpose; hence engaing in its natural taspih!

After this, you say:

“Sami Allah liman hamidah”

“Allah is the perceiver and Hamd belongs to Allah…”

That is, every action I take is a result of the Divine Power that is in effect, Allah, as the Absolute Executer, knows what I do, for it is Him alone who determines this in His Knowledge… 

Then you stand up and go back to an upright position again. And while in this upright position you say:

“Rabbana lakal hamd” in short, or the longer version “Rabbana lakal hamdu kama yanbaghee lijalali wajhika wa liazimi sultaniq” which is the version most frequently preferred by the Rasul (saw).

You are not to prostrate until you fully stand in an upright position and only then should you utter this tasbih, which means:

“Only My Rabb can evaluate His Perfection, Magnificence, Wisdom and Comprehension. None has the capacity to understand His Power and Infinite Potential and Unlimited Capacity!”

Then you claim, “Allahu Akbar” and kneel down to “prostration” …