First, i was surprised that i was able to perform 3 sets of 1 rep resting 30 seconds for rings mobility. probably i got the necessary strength before today, it was a technique matter. No problem my joints will thank me for that!
Yeah sounds strange, but i'm using foundations approach everywhere.
the most difficult thing is try to balance during the inverted curl where, for necessity, is useful some retraction. i got it clear at the 3rd set. No assistance where done by my brother , he was simply watching that all was well.
Advertisement: if you can manage one rep don't try it after the mastery of HBP PE19. 
Now it's time to practice, look out in handstand (i previously got the rings HS) and multiple reps.
I think there are no secrets. 4 years of gymnastic training, step by step, progression over progression.
the very good aspect of that is my body is so prepared that i'm not feeling any type of soreness or pain in forearm, biceps, shoulder, back.