Lie on your back with your partner standing firm near your head so you can reach up and hold onto his ankles. Suck in your abs and scoop your pelvis up to press your lower back flat against the ground. Now tighten your abs even more and raise your legs towards your partner, keeping your legs straight and back flat on the ground. When your legs are fully extended, your partner can push them back to start. Do all reps, then swap roles.
Don’t let your feet touch down between reps.