Cluster sets break a 'set' into 2-5 smaller sub-sets with 10-30 seconds of rest between, while still counting as one set.
Mechanism — Short intra-set rest restores ATP-PCr and clears local metabolites without dropping HR. Result: more total reps at the same intensity, or higher intensity at the same total reps. Strength and power applications especially.
Use case — Power output preservation in heavy training (e.g., 5 sets of 3+3+3 at 85% with 15-sec intra-set rest). Builds maximum strength without grinding reps.
Caveats — More complex programming. Don't use for hypertrophy where time-under-tension is the driver — there, continuous sets win.