The Axial Twist:
Kick propulsively while rotated at about 45 to 60⁰degrees, keeping both hands at thighs breathing continuously. The goal is to keep the kicking feet and upper butt cheek at the surface, breathing continuously with one goggle lens UW. Fins work great.
Head-up freestyle with fly kick:
I use a snorkel and fins, the aft end does its action naturally, but for the upper body it's a vigorous game of catch - pull - catch - pull. Of course fins and snorkel work good doing these.
Wear snorkel and use fins to get you moving at a sufficient speed with arms back (hands at thighs). Do a full stroke: Rotate and recover up arm (but keep other hand still at thigh) counter rotate and catch following through with a pull but keep hands at thighs at end of stroke kicking several times, still rotated, ready for next arm. This drill helps me connect the energy transfer from rotation to shoulder to catch, as well as pulling the hand in a proper path.