Halts attacking biter groups and lets them fester until a certain time when they all attack at once. Use the periods of safety to build up defenses, or clear out the nearby nests/groups before their next big assault. Also adds a new combinator found on some spawners you can use to check the status of the next wave and a non-lethal laser turret you can unlock sometime before you get artillery to mark those spawners for your defenses to ignore, just watch out for splash damage.
Mods introducing new content into the game.