What does a push-up bra do?

A push-up bra has foam padding with graduated thickness inside each cup. The thickest part of the foam should be concentrated underneath the bust, and it will decrease in thickness, sloping to its thinnest at the top of the bra cup.

The thick padding beneath the breast is what lifts, or pushes up, the breast tissue to give that enhanced cleavage effect that push-up bras are known for.