A snippet from the bassplayer.com site:
As a result, fans finally felt the full force of his techniques: standard fingerstyle and pick playing, a technique he calls crab-claws, backhanded chords, string pops and smacks, harmonics, and left-hand hammer-ons. Then there's his impressive typewriter technique, a forerunner of today's bass tapping, in which he strikes the strings at the base of the neck with his four right-hand fingertips in quick, typewriter-like motions, enabling him to play rapid triplets and various other drum-like devices.
A snippet from a BBC website:
Songs like Ten Little Friends have Entwistle masterclassing in the typewriter bass technique he created years before funk-pioneer Larry Graham started slapping.
And there's a whole tutorial on it from the man himself on his old Hot Licks video from the late '80's...man, he was really smarmy in that one.