I have one I’ve been using for years on the road. Haven’t installed it on a MTB though; haven’t had the desire to. But it’s a battery-powered device on your non-drive side chain stay that sends a signal to your computer based on magnets on your rear wheel and non-drive crank. It’s great on the trainer, as you can turn off gps (in the settings) to save battery power, and then get speed and cadence info. Additionally, it’s nice off the trainer because speed/distance are more accurate with this. And if you’re in dense forest on the MTB, losing a signal every so often won’t affect your final distance measurement (as the speed/cadence sensor of course keeps running regardless of GPS signal).