Words drift down the screen and you type. A completed word disappears, an incomplete one keeps falling, and the counters along the top track your speed and your accuracy while you are busy not looking at them. Space pauses. There is a tips option that shows the next word coming, at a small cost to the score.
It gets faster by level rather than by density, so the pressure is on your hands rather than your eyes, and the run usually ends because two words became urgent at the same moment and you started the wrong one. It is a typing test that has stopped pretending to be a typing test, and it is a considerably better way to spend fifteen minutes than an actual one.