Ska upstroke — the fast ancestor of the reggae skank. Guitar and piano play a short chord on **every off-beat (the "and" of 1, 2, 3 and 4), and on guitar every one is an upstroke**: the pick travels down on the beat without touching the strings and catches the top strings on the way up, which is what gives ska its clipped, bouncing sound. Under it the bass walks straight quarter notes like a jazz bassist, and the drums play 2 and 4 — half the tempo of the skank, twice the energy.
How it sits: compare with reggae: same off-beat idea, but at 150 instead of 80 the off-beats come twice as fast and there is no double chop, just one even upstroke per beat. The I–vi–IV–V (C Am | F G) is the classic two-bar ska/rocksteady turnaround.
Listen for the bass and the chop never coinciding — the bass is the only thing on the beats. Keep the chords tiny (a sixteenth) and dry.
Practice: strum muted down-up continuously, then unmute only the ups. Authored from standard practice — no transcribed source in the workspace.