Lower Ab Criss Cross
To do the Lower Ab Plank Criss Cross, lie on your back with your arms down by your sides or placed behind your head. Do not put them behind your low back or butt. Raise your legs straight up toward the ceiling and press your low back down into the ground, engaging your abs.
Then lower your legs down so they are only an inch or two off the ground and cross one leg over the other. At a controlled pace, then spread your legs out wide before bringing them back together and crossing the other leg over on top.
If you feel your low back taking over, raise your legs up higher or slightly bend your knees as you criss cross. You can even bend your knees to 90 and simply open your legs apart and bring them back together even to modify.
You want to modify to make sure your abs are working instead of putting your hands behind you.