Legs up, sweep them side to side like a wiper.
Keep both shoulders pinned to the floor and move your legs slowly.
The windshield wiper is rotational core control at its purest. With shoulders pinned wide as an anchor, your core has to lower both legs to one side, stop them before the floor, and drag them back through centre, resisting rotation as much as creating it. It sits a clear level above crunches because the challenge is deceleration and control, the qualities that actually protect and strengthen a midsection.
Bend your knees to shorten the lever and reduce how far you lower to each side. Bent-knee wipers with full control build toward the straight-leg version far better than sloppy full-range reps.
Only as low as you can while both shoulders stay glued to the floor. The rep is defined by the shoulder that wants to lift, and once it does, you have gone past your current range.
Same idea, much higher tier. The floor version lets you master the rotation with your back supported, and it is the mandatory stepping stone before attempting wipers from a bar.
