Instant Action is in the original F.E.A.R. made by the original crew. It is also in the expansions, it WAS a staple and hell, if it wasn't for Instant Action, there probably wouldn't be a Terrorist Hunt in R6V2. It's basically a map with enemies that spawn under certain conditions dependin' on how you move or on some maps just over time. Think single player but MP maps, though F.E.A.R. made specific maps just for IA, but don't think of it as MP with bots. You are given a time limit, you set a difficulty, and you try to defeat everything on the map without dyin'. You also only get one life to do this, maybe more on easy modes but I've never done them on easy. It's probably best to call it a time trials mode, I guess. Anyway, it's really fun and ups the longevity of the game. I still play F.E.A.R. because of this and I got the game when it came out on 360, though now I play the PC version.
It should also be mentioned that this is not the cake walk a normal SP game play would be. Time aside, you're ammo, med-pack, and armor is rationed very sparingly and, well, F.E.A.R. is already known for its difficulty among FPS games, so I'll let your imagination run with how hard IA can get. Most of the spawns are very, very inconvenient, especially the "boss" spawns. Shit can get nuts, fast.
If you're not a F.E.A.R. Vet, you didn't loose anything, it's like never playin' R6V2, but buyin' the third one, which for sake of argument doesn't have Terrorist Hunt, and not carin' because, hell, you don't even know what it is.