what can I say? Just don't lie too open when lying down. Just do it hard. Bend your knees about 30 to 50 degrees. Hands on your head are also very powerful. The distance between your arms is very small, about the width of your face. Put your hands behind your head and hold the upper part of the back of your head. When you remember, push down with your hips and bounce like a spring.

Posture is probably a personal habit. I almost like doing sit-ups like that. I do 53 in about 1 minute.