Players subdivide turns into impulses alternating between players which repeat until both players pass (or in some cases a sunset die roll ends the impulses catching one or both players off guard). In those impulses a group of units is once activated or gets to act collectively before being marked spent. However instead of the activated units being grouped by a certain radius from a leader the units activated are in an area (and thus the need for the impulse system to have areas, not hexes). The areas exist to define scope of activation on an impulse (as well as restrict what can be done on that impulse with respect to attack and movement range). Thus each of a players groups of units each acts once by means of small alternating impulses instead of the traditional all my units then all your units. Finally before the next turn of impulses spent units are reset regaining the ability to act.
Historically this began with Storm Over Arnhem (1981) part of the "Storm Over" area impulse series