An exercise that is done wrong 90% of the time. In order for this exercise exercise to be effective there has to be a posterior tilting of the pelvis. It’s not just about flexing your hip and knees, which I see done a good majority of the time. Rotate that pelvis back. Aim to bring your knees to your chest. Also, this is an exercise that should be worked up to. A typical progression would be reverse crunches to incline reverse crunches than to hanging knee raises.