As the handler, my job is to keep the horse's attention on us. I remind the horse to stand, quell minor fidgeting, discourage leaning or nipping. I keep the horse stood up properly, avoiding walls and farrier tools. Mostly, I use my voice, rattle the shank, or a hand on the neck. Any major discipline is the farrier's job-- a whack to the belly, whatever-- because he is then aware of the horse's impending reaction. I'm a very active participant in the process, but my farrier's safety is my absolute #1 priority.
Not all horses need "discipline." I know some horses who act up, won't stand with a lip chain or twitch... but a small bucket of grain makes them still as a statue. It's my job to stand the horse, however I can... but if a CTJ meeting is required, the farrier is god.
“A clever person solves a problem. A wise person avoids it.”
? Albert Einstein