It is a very tight fitting shirt. The non stretch ones (denim, canvas) compress your body as the bar comes down to the chest. Your body wants to return to its normal shape and attempts to do so as you press. These shirts help a lot off the bottom but don't do anything after the first half of the press or so.
Poly shirts stretch somewhat giving you the rebound of the material, in addition to the body compression you get from a denim shirt.
Hope this helps. go to google and put bench shirts and titan in there, you will find your way to titan's website which shows what a bench shirt is, and why it works.
B.