Stabilise yourself in the side lying position, using your upper arm and under leg
Raise your upper leg till it is parallel to the ground and hold isometrically for the specified amount of time.
Feel your bum (glute med) and the lateral line of your leg worked
Przewodnik Ćwiczenia Abductor Leg Raise Side Lying Isometric - Trening Glutes | MuscleWiki