This is a movement that is more difficult than it seems. Seated leg lifts will help develop short range hip flexor strength needed in CrossFit workouts and for Gymnastic skills.
Start seated in a pike position, reach towards your knees and lift both of your legs at the same time. (to increase the difficulty reach forward). Some variations include single leg raises (raising one leg at a t time) or even try doing them in a straddle position. Increase reps and duration while performing until failure.
Give this drill a try. Let us know what you think!