This is a simple but effective exercise for the abs.
Sit down on a bench. Legs should be slightly bent at the knees and hanging. Hands are placed on the bench.
As you inhale, stretch from the starting position so that the legs are almost extended and the back is as close to the bench as possible.
As you exhale, slowly pull your legs to your chest, while bending them at the knees and bringing your chest to your legs.
Don't try to jerk up, you can get injury. And slso you won't put the proper load on the working muscle.