Chest Dip

Trains the Abdominal Head.

How to do it

Set parallel bars at roughly hip height and grip them with your arms straight, supporting your bodyweight.
Cross your ankles behind you and lean your torso forward over your hands. This lean is what separates a chest dip from a triceps dip — everything else is nearly the same.
Let your elbows travel slightly out from your body rather than tucked tight to your ribs.
Lower yourself under control by bending the elbows, keeping the forward lean, until you feel a stretch across the lower chest and your upper arms are roughly parallel to the floor.
Press back up by driving through your hands, keeping the torso leaning forward the whole way.
Stop just short of locking the elbows at the top to keep tension on the chest.

Staying upright. An upright torso makes this a triceps exercise — which is a fine exercise and a different one. Leaning the chest forward over the hands is the whole mechanism that puts the load on the lower chest.
Dropping fast and bouncing out of the bottom. The bottom of a dip is a deep shoulder position, and dropping into it under bodyweight is the least controlled way to arrive there. Lower deliberately.
Going deeper than the shoulders comfortably allow. Upper arms roughly parallel to the floor is enough for the chest; more depth adds shoulder range rather than chest work.
Expecting dips to reshape the bottom edge of the chest. That line is set by where the muscle attaches, which training does not change. What training does is make the muscle underneath bigger, which is worth doing on its own terms.