stretch action
The phenomenon where the section of shirt fabric between a woman's breasts becomes stretched taut, as evidenced by several prominent horizontal "stretch folds". It occurs only when the right combination of shirt tightness, breast firmess and breast size is met. Stretch action is often displayed prominently in over-the-top anime busts.
Guy 1: Hey, check out the stretch action on that girl over there.
Guy 2: Nice!
Guy 2: Nice!