Ab Crunch Machine

How to adjust

Set the seat height so the machine's pivot sits at your waist — roughly level with your navel or the bottom of your ribs when you sit tall with your back on the rest.
Set the chest/shoulder pad (or the overhead handles) so the pad meets your upper chest and your shoulders are not pushed up toward your ears when you sit tall.
Set the load with the pin on the weight stack, starting light to feel out the range before adding weight.

Sit into the machine, chest against the pad or gripping the handles above your shoulders, bracing your feet under the footplate or hips on the seat.
Exhale, curl your torso forward and down by contracting the abs.
Pause for a beat, inhale and extend back to an upright seat over 2-3 seconds, without letting the weight stack drop and bang.
When you can no longer curl while keeping your chest on the pad and your feet braced, end the set: extend back slowly until the stack settles all the way down, let go of the handles, and only then take your feet off the footplate and stand up out of the machine.

Brace your feet or hips against the footplate/seat before curling forward, since without a stable base the resistance can pull you awkwardly forward off the seat.
Don't yank with your neck or push your chin forward to move the weight; keep your neck in line with your torso and let the abs do the work, dropping one weight increment if it feels too heavy.
Return to upright under control; when your strength runs out, lower until the stack settles all the way down before you let go, instead of releasing mid-rep and letting the pad snap back.