[Various helpful hints about care of meters and detecting malfunction of them]
LRH had a "beep meter" which you could influence with energy flows. You can do this to a person's body, too. The "beep meter" detects pain in the body; when held in the area of pain, it goes "beep". A person can do it remotely by "seeing" a black area in the person who is holding it and turning it white. Someone who isn't a scientologist can't do it, just because of not being in good enough shape -- not having enough "horsepower".
As a thetan, you can knock the needle with a beam. It looks like a body motion, a jerky tick. If the PC is influencing the meter, the read will be latent. He can't hear the command and put the meter into action as fast as the reactive mind can.