Lots of arm muscle.
May I suggest training in the gymnasium 3 or 4 times per week to include chin-ups, curls, push-ups, parallel-bar dips, etc.
Technique also helps a lot.
Personally, I like to pull toggles to shoulder level, then rotate my elbows up so that I am using more muscles (pectorals, latimus dorsi, trapesius, etc. to finish the bottom half of the flare.