High you want the fan to go, or there are temperature sensors embedded in it so it can decide how much fanning you're likely to need. i thought it was going to get caught up in my hair, actually. amazingly, it doesn't seem to — although if i move my hair out the way, i can obviously feel it blowing a bit more. would i really wear this? well, maybe if i was in a hot country it might feel quite nice, and i suppose it does just look like a pair of headphones around your neck. but still, the concept itself does feel a little bit over the top, to me — although maybe i wasn't born to be this coo...