I was so hoping to not here the name of Derren Brown on this forum; every F*ing newbie to magic that's from the UK wants to be the next Derren or worse, Criss (I'm a middle-aged Goth Child)Angel.
I do believe most people that had even preliminary Psychology classes in school is familiar with ideomotor syndrome, I even teach people with mobility issues how to use this technique in steering their wheelchairs (the joysticks really are that sensitive). BUT, there is another reality here that I bring up only because of my personal work with dowsing. Simply put, there IS a lot more to it than what the analytical want to use as a cover-all explanation; not in the case of Derren or any other Psychic Entertainer type character (including yours truly) but when working with someone that really is "in tune" with things, pendulums, planchettes, rods and yokes can go a bit crazy... I've had
water twigs pull me hard enough to fall and seen them literally pulled from a persons hands and stuck into the ground.
Please understand what I'm saying here; I'm not saying that ideomotor isn't involved. In the majority of cases it is very much what's afoot but in certain rare settings very dramatic things can happen when working with such systems.
BTW... I do make nice wood planchettes if anyone wants to go ghost hunting