The short answer is technique and respect.
Any half decent instructor who teaches head motion will start with techniques for safety. Anyone who doesn’t have some idea of good safety practices shouldn’t be attempting head motion. A follow has to care enough about herself to refuse to do something that she doesn’t know how to do and that might therefore lead to injury.
It’s the responsibility of the lead to gauge the follow and not try to force her to do something she isn’t comfortable doing. Many, hopefully most, people will tell you that “Rule Number 1” is zouk is “Don’t hurt the follow.” This is where the respect comes into play. You have to care more about the health and wellness of your follow than about trying to look cool on the dance floor or about impressing your follow or about forcing her to do what you want to prove you’re in charge. A mediocre lead can do cool stuff. A good-great lead can connect with the follow regardless of her skill level and have a great dance without trying to be flashy. (Opinion: Zouk is more about connection than it is about the moves you do.)
Too Long ; Didn’t Read: Find someone who knows what they are doing and have them teach you how to be safe.
We just started having workshops in my city over the last year… and we have not touched on head movements in group classes. It’s more important to work on steps and Lead+Follow for a while. What I can say is after a few private lessons, I do know it’s not so much a head movement as it is positioning the shoulders and letting the direction of the head follow, keeping the chin tucked at a certain degree…. little things!
FYI: It’s not actually movement from the neck. It’s an illusion created by hair and shoulders and chin direction/following… I highly reccommend seeking private lessons for 1×1 technique assistance.