Bicycle crunches get harder when the ball makes you balance.
Twist from your ribs, not your neck, and keep the ball steady underneath you.
Doing bicycles with your lower back supported on a ball changes the game twice, the ball lets your torso extend slightly beyond flat so the abs work from a longer stretch, and its instability forces your core to steady the surface before it can produce the twist. The elbow-to-knee rotation drives the obliques while the deeper crunch pattern hits the rest of the trunk. It is a natural progression once floor bicycles stop feeling like work.
Two reasons, the ball adds a slight extension at the bottom so your abs travel through more range, and it wobbles, so your core spends energy stabilising before it even crunches. Same movement, more work per rep.
Under your lower back, roughly the belt line. Too far up your shoulders and you lose the stretch, too far down and your hips do the moving instead of your trunk.
You are pulling your head with your hands. Keep the fingertips light by your ears, lead the rotation from your ribs, and let your head simply come along for the ride.
