Marley

A strong knee pit grip is needed before working on this move, and you'll see several related tutorials to help you prepare for or improve on this trick. You don't need to pull on the foot to be or feel secure in this pose. If you rely mostly on pulling on the leg, you can injure your knee. It's much safer to wait and build up the strength of your hamstrings to hold this pose.

Gemini

Side leg hook crunch

Planks

Spinal contractions

Strengthens: HAMSTRINGS / GLUTES / OBLIQUES / ERECTOR SPINAE (THE BACK)