Crunch on the dome for deeper range.
Let the dome arch you back at the bottom for more range than a flat crunch.
The dome fixes the standard crunch's biggest flaw, no range, by arching your lower back over its curve so your abs start each rep from a genuine stretch and work through a far longer arc to the top. That stretched starting position is exactly what floor crunches can never offer, and it is where the extra growth stimulus comes from. It earns its place as the straightforward upgrade for anyone whose floor crunches stopped producing anything but neck strain.
For range of motion, clearly, the dome lets your spine extend back at the bottom so the abs work from stretch to full contraction instead of just the top half. Floor crunches remain fine, the dome version simply gives each rep more to do.
Under your lower back, roughly the belt line, so your torso can drape back over the curve. Too far up under your shoulders and you lose the stretch, too low and it just pushes your hips around.
No, curl your ribs toward your hips until your abs are fully contracted, usually with your upper back just clearing the dome. Rising to a full sit turns the top half into hip flexor work and rest time for your abs.
