I've watched the geysers for longer than I should admit, and there seems to be no sensor, no rhyme or reason. It just depends on when you're going over it. They don't go off every moment, they might be timed, I don't know, but (when they are actually ON) some people just don't get hit with it at all, some people have their rafts going over it as it's rumbling but it doesn't hit them when they go over it, some people run right into a spouting geyser, and some poeple have it rumble under then it goes off just as they are coming off of it, which probably causes the MOST fun geyser action to watch (and experience). Just when you think you're safe!
But if you have SEEN the geysers going (at the very end of the ride, after you've gone down the last hill where you generally turn all around, when you think you're just on your way back to get out of the rafts) and not gotten hit, then you're very lucky or unlucky depending on if you wanted to get wet.
Sometimes the geysers go off and will only hit half the boat, sometimes it hits just one person, sometimes the whole boat...it's just really random and I'm sure the physics of water under pressure has a lot to do with it....