These are often referred to as chicken fillets due to their shape. The silicone is quite dense making for a firm fillet. This style of insert is very good for giving you a real boost if used from the underside of the cup. It can also be used successfully on the offside to provide you with a decent cleavage.
They are most popular with people with a small to medium cup size although some people with a larger cup size use them for the cleavage boost.