One drill that I have found quite useful for simluating Homnas is:
-on the end of a 20cm (forgive my metric here) safety mat facing the length of the mat, gymnast is in front support...coach lifts feet so gymnast is horizontal in front support (slightly arched)...coach rapidly lifts heels three times ("1...2...3.."), with the gymnast staying in a slightly arched position...on "3" coach rapidly lifts heels upward (thus, flipping the gymnast forward)...gymnast pikes while rotating (this will come quite automatically) and lands sitting in l-sit position on mat.
After a gymnast masters it on the 20cm mat, you can move it to a low set of rings (typically, 5 to 10 cms higher than the safety mat underneath the rings)...this is helpful in stressing the importance of keeping pressure on the rings, and better simulates having to finish in support.
It sounds a little crude in type, but it works quite well. If you have any questions, please contact me and I can see what I can do about posting a video.
Once the gymnast has done these drills for a while, take it up to regulation height rings. Simple tap the gymnast a little bit on the upward phase of the swing, if needed. Watch out for any chicken winging...
