This exercise is to teach how to maintain a hollow body position and improving core stability. for the first 4-6 reps (when lowering legs down), exhale completely to improve abdominal activation and to connect mind-muscle.
It should feel as you are tilting the pelvic forward and flatting the back against the ground. Make sure to maintain this completely at all time and all through the movement.
The movement itself is by moving the legs through the hips and keeping the knees in the wanted angle.
In order to make this exercise more challenging, straighten the knees until reaching the maximum load for abilities.