This is my single favorite lower ab exercise. The key here is control. A large part of the difficulty comes from slowly lowering the weight. In the first half notice the great range of motion I have. As I bring my feet back I maintain the same angle at my knees. This forces all the work into my abdominals as my lower body stays totally ridged.
In the second half of the video you see my form change. Instead of dropping the weight I change to the easier version by allowing my legs to fold together as I curl them back towards my body. Both are correct but the first is more advanced and puts a greater emphasis on the lower abdomen. In both movements you will have to grab onto something sturdy.
(the other ankle strap was missing or I would have used it)