This concept is derived from the fact that man's progression towards Hashem begins with the Aleph which is the lowest number of the Aleph Bais then progresses towards the Sav which is the highest number in the Aleph Bais. Hashem on the other hand begins with the highest number of the Aleph Bais, the Sav, and progresses towards man at the other end of the Aleph Bais. So Gematria numbers that begin with high numbers on the right and move in progression to lower numbers on the left represent man's ascent towards Hashem. Man moves from lower numbers on the left towards higher numbers on the right which are represented by Hashem. The word Pesach is a classic example of this.
Man progresses towards Hashem on the right.
The first man Adam was formed from red clay, dust. When we approach Hashem we come humbly and contritely before Him, from the left towards the right. He is our Maker and we are His Creation. Pesach spelled in reverse order is Chah Sawf meaning potsherd, clay. We humans are constructed of this clay, of this potsherd.