An L hang is still a worthwhile exercise, as it is essentially an isometric version of the hanging leg raise. As for the hanging leg raises, you need to stabilize your hips from swinging. When you raise your legs up, try to keep your hips from moving forward. When you get to the L position, pause in this position if you need to in order to stop any excess movement. Once you are still, lower your legs back down with control. With practice, you will be able to do these steady straight leg L raises with more precision. Once you can comfortably do that, you can start working on the full hanging leg raise. From the hanging L, tuck your knees to your chest, bring your hips up as you lean back and unfold your legs so that they are pointing up and your shins are against the pull up bar. From here, slowly lower down to the L then to the dead hang. Over time, you will be able to straighten your legs out and go directly from the L to the top position.