Stretch your legs as you say: in the process of lifting your legs upwards, your waist should stick to the ground as much as possible. Only when the angle between your legs and your body is less than 90 degrees can you make your waist leave the ground. At this time, the abdominal muscles are tightened and the waist tightening is normal.

But if the angle between the legs and the body is less than 90 degrees, the waist will leave the ground, which is not standard. It is recommended to watch more training moves, or consult a fitness instructor or friends around you.