Description from Chain Reaction 3.0 rules, p2
"Traditional games use a turn sequence known as “IGO, UGO”. This means that I move my figures, fire, we do some melee, and maybe you do a morale test or two.
THW uses what is called the Reaction System. In this system your side activates and you move part of your force. That triggers a reaction from parts of my force. I immediately react and this may cause you to react in kind."
System used by Two Hour Wargames and derived games, such as All Things Zombie, All Things Zombie: The Boardgame, 5150, Adventures in the Lost Lands, and others.