Abs, Core and Explosive Power. That’s why you do sit-up stand-ups.
Lie face up with bent knees. Anchor your feet under a heavy bench or dumbbells.
Sit up with enough force to generate momentum to stand up. Sit back down then lay down and repeat.
The benefit of using a bench is that you can grab onto the bench to assist you into the standing position if you need the extra oomph to get up.
If you have a partner they will put their knees on your feet while they hold onto your calves. Watch out for a head butt if you actually come up.
One of the benefits of having a partner is that they can offer out a hand to pull you into the standing position if you cannot generate enough force by yourself to stand up.
If sit-up stand-ups don’t come easily for you then try this:
Push your heels into the ground and point your big toe and keep practicing!