Plunge bras are distinguished by the part of the band that joins the cups. The defining fact is that if it joins below nipple level, then it's a plunge bra. Plunge bras are ideal for wearing under low-cut tops. Many plunge bras are padded in order to push your breasts together to maximise your cleavage. The plunge bra, won't give you the support and security you might normally want, but sometimes the WOW factor is worth the sacrifice for that special occasion.