Rocksteady — the short Jamaican bridge (roughly 1966–68) between ska and reggae: Alton Ellis, the Paragons, the Techniques, Duke Reid's Treasure Isle and Coxsone Dodd's Studio One. Legend says it slowed down because one summer was too hot to dance ska.
How it sits: the tempo halves the ska feel, so the off-beat chop now lands on beats 2 and 4, one dry downstroke on guitar. The piano chops with it and adds a quick second chop on the "and" — the seed of the reggae bubble. The drums already drop beat 1: kick and cross-stick together on 3, hi-hat eighths over the top.
Listen for the bass: rocksteady is where the bass becomes the melody. Here it moves root–third–fifth up into each chord and walks the I–vi–IV–V, the instrument everyone hums afterwards.
Practice: play only the bass line until you can sing it, then add the chops on 2 and 4. Compare ska-upstroke (twice as fast, every "and") and reggae-one-drop (sparser bass, the bubble).