In the early days of Call of Duty, Activision reportedly tried to out-do Counter-Strike. Activison's FPS series began to hit its stride once the team abandoned this approach and tried something else, though, and it involved killstreaks. Speaking to IGN, designer Mackey McCandlish, who worked on the early Call of Duty games, said the team decided to make the best possible version of Team Deathmatch instead of trying to top what Valve was doing with more objective-based modes in Counter-Strike.

"The interesting choice going from Call of Duty 2 to 4 is that we finally gave up on the idea that we were making a better version of Counter-Strike," McCandlish said. "We said, 'You know what, our players just want to play Team Deathmatch. Is there anything we could do to make Team Deathmatch something we want to play too?'"

So McCandlish and the team thought about how they could "carrot" players, or find a way to encourage players to stay alive for a while like they would in a single-life mode in Counter-Strike. The solution was killstreaks, which are unlocked every few consecutive kills.

"The way to get there was the killstreaks, where, 'Hey, I've got two or three kills, now I have a reason not to just go run into the next person.' There's a little bit of stakes there," McCandlish said.

Killstreaks are now a cornerstone of Call of Duty. They encourage players to try to stay alive so they can unlock more and more powerful rewards.

One of the new killstreak rewards in this November's Modern Warfare III is the Guardian-SC. This emits a non-lethal beam that produces effects similar to a stun grenade, which means slowed movement and blurred vision for anyone near it. It's particularly useful in modes like Domination where a team needs to hold a particular location.

Modern Warfare III launches on November 10, but the game's campaign mode will be available to those who preorder starting on November 2. See GameSpot's Modern Warfare 3 preorder guide to learn more.