One foot down, crunch hard, and let the dome test your balance.
Keep the ball still by bracing your core, do not push through the planted foot to rock.
With your lower back on the dome and one foot floating, each crunch starts from a stretched position over the curve and finishes with your abs pulling the ribs toward the hips while the core fights to keep the dome still. Losing one foot of support means the midsection stabilises as well as flexes on every rep. Trained one side at a time, it earns a place for adding both range and balance demand to a movement most people have long outgrown on the floor.
Two planted feet let you brace against the floor and rock the dome to help the crunch along. One foot up removes that cheat, so your abs both lift the torso and hold the ball dead still.
No, finish your reps with one leg lifted, then switch the working side. That keeps the stability challenge constant instead of resetting every rep.
Your spine extends back over the curve at the bottom, stretching the abs before they contract. The floor cuts that range off, the dome gives it back.
