Smith Machine Decline Press

Trains the Abdominal Head.

How to do it

Set a decline bench under the Smith machine so the bar lands over your lower chest.
Hook your legs under the pads before you lie back.
Set the safety catches just below chest height.
Take an overhand grip a little wider than your shoulders and twist the bar to unhook it.
Lower to the base of the chest under control, then press back to almost straight arms.
Re-hook the bar before letting go, and unhook your legs only after that.

Getting the bench position wrong and trying to adjust mid-set. The bar's line is fixed, so the bench decides where it lands — and on a decline, sliding the bench while hooked in is not an option.
Skipping the catches because the machine has hooks. The hooks catch only if you can still twist the bar, which is the thing a failed repetition prevents.
Lowering to the mid-chest as on a flat press. The decline puts the bar low, and forcing it higher takes the shoulder into the position the angle was chosen to avoid.
Unhooking the legs first at the end. The pads are what stop you sliding head-first, and they matter right up to standing.