Lie on your back, chins and thighs are in 90 degrees or slightly more, also your thighs and your upper back are 90 degrees or a little bit more. Arms are next to your body. In order to avoid pushing with your arms and really letting your core work, keep the palms facing upwards. Shoulders are touching the ground. Engage your core. Keep the angle in your legs and move both legs towards the ground, gently touch the ground with your heels (do not rest on the heel), move back into the starting position but do not bring your knees closer than 90 degrees to your chest in order to stay under tension all the time. During the whole movement, squeeze your nable in. The slower you do this movement, the harder it gets.
Normal burpee. No explanation needed 😉