The right biker zai, after pull his shirt over his head, go and kiap his neck muscle until he cant move lol.
The Vulcan death grip!
Seriously though, just gripping the back of a person's neck won't cause enough pain to subdue a person.
We're not cats who learned to keep still when mommy cat bites the nape of our necks to carry us.
What actually happened is left guy was pulling the loser's hair with his left hand, and using his right hand on the guy's exposed neck to keep him off balance, and let the loser know his vulnerable neck is also being threatened.
Loser's center of mass was too far forward and his legs were blocked by the bike. So he can't stand up straight since his feet can't move further under him to lend strength and balance.
Nor can he pull his body back enough for his feet to be under him since his hair was being pulled.
Loser also gave up the fight once his jacket was pulled over his head and restrained his arms' freedom of motion.
Knocking the mental desire to fight out of an opponent is also a good way to end a fight.
A more evil and aggressive person would have used both hands to grab the loser's hair and repeatedly slam his soft face into the hard metal bike. At full force. Until his nose is broken, some teeth fly loose, torn face bleeding in an unrecognizable mess of flesh and jaw broken.
A killer would just use his leverage on the loser's head, use his right hand to do an overhand grip of the loser's chin, then quickly pull up to twist the head sharply to snap the neck, severing the weak C2 cervical vertebra.
So yah, this is small potatoes and a benign fight.
Also, remember to always control one's temper and don't fight. One never knows if the unsupposing opponent is a trained CQB expert reacting on pure reflex...Or is a siao lang...Or both...
In the heat of things, mental restraints from the fear of consequences can evaporate instantly.