I join up with all new-ish horses I dont know that act dominant towards me or another person. Its like majic, I was working with this mare and she was being very piggish: bucking, kicking, pinning her ears, etc. after a lot of fighting I lost my temper and untacked her and took out the saddlebreds' whip (very scary whip) and made her gallop for about ten minutes straight. At first she was like "Hell YEAH!" >
and took off for about five laps. Then she started getting tired and tried to trot. I made her gallop even faster. After a few more laps, still at the gallop, she licked her lips. I let her whoa, she dropped her head and walked up to me with her nose touching the ground. She let me pick up all four hooves, disengage her front and back ends, and lead her without touching her! I even lounged her without a line! (as in she was circling me, not lapping the arena) it was like she was a different mare.
The funny thing about Joining up is this:
There were two other people in the arena with me doing the same thing I was to her, but she chose to join up with me. She still was being piggish about everything with them, but with me she was very submissive. So, joining up helps YOU become dominant with the horse, but not other people.