What is multi-finger tapping? It's pretty self-explanatory, as it simply involves the use of multiple tapping-hand fingers tapping through a passage. Its existence is for practical reasons, as sometimes it’s difficult to only use a single finger to tap. And of course, no one can deny that multi-finger tapping looks pretty cool too!
Alright, let's check out the exercise. Tap your middle finger on the first High E string note (19th fret). Then immediately after that, pop your middle finger off the E string and tap your index finger on the B string note (18th fret). Make sure this movement is swift and accurate, and almost twitch-like.
Once you get used to this exercise, try to tap it with only one finger. Not that easy is it? You can see that without using multiple fingers, this exercise would be nearly impossible at fast speeds.