In one of the interview with Nolan he says he has full body spasms when he sits in the chair and those spasms take him out of position from being able to use the mouth stick controller. With neuralink he doesn't need intervention by someone else post spasms to continue.

Definitely enough to be jostling the head, but he didn't get into explicit detail of how serious they are movement wise.

Edit: side note, makes me wonder if they're a build up of spinal signals and the cord briefly connects and suddenly a pile of commands go through and he spasms.