Watch the The iii-vi-ii-V Progression online guitar lesson by Fareed Haque from Jazz Comping Survival Guide

In the video I mention that iii-vi-ii-v is not that different from I vi ii v, but that it opens up possibilities for "turning around" iii (say Bm7 in key of G) is basically G maj 7 w/b in bass (known as first inversion, fifth in bass is second inversion, and you guessed it, 7th in bass is called third inversion) but thinking iii instead of I means we can use principles of chord sub to lead for exmple to Bm with F#7, so we might get G-E7-Am7-C#m7 F#7 (instead of D7) Bm-E7-A7-D7 etc. So leading to iii can bring some other chords, allowing us even more variety in turnarounds.