what it's called, but basically he records a refrain to one of several "tap" positions and then he can turn that refrain on and off for the remainder of the song as needed. He can wind up with several refrains harmonizing with whatever he's currently strumming.

It's obviously not possible for him to be playing all of those notes at the same time, but they are all of his notes that you watched him strum and it's certainly a talent to tap everything on and off just right to get the perfect harmony (as well as keeping the beat so that it all stays timed together).