Reading the four digits
Digit 1 — the eccentric (lowering) phase, in seconds. Digit 2 — pause in the stretched position. Digit 3 — the concentric (lifting) phase; “X” means move as explosively as possible. Digit 4 — pause at the top.
So 3-1-1-0 on a squat: three seconds down, one second in the hole, one second up, no pause at the top.
Why it matters
A controlled eccentric increases time under tension and is disproportionately responsible for the muscle-building stimulus — and for soreness. Pauses remove the stretch reflex, making the lift honest at the exact point most people bounce. Explosive concentrics recruit more of the fast motor units that strength work targets.
Tempo is also the cheapest overload lever there is: same weight, same reps, harder set — useful when equipment jumps are too big (see progressive overload).
Typical prescriptions
Hypertrophy work commonly runs 3-0-2-0 — controlled down, smooth up. Strength work runs 2-1-X-0 or similar — braked eccentric, explosive intent up. You will find the prescribed tempo in the programming table of every exercise in the library.