When you are playing a team game you will be able to knockdown enemy players when their health reaches 0. When they are in a knocked down state they won't be able to do much but crawl around and hope one of their teammates will come and revive them.

When you approach a knocked down player you will have the option to shake them down. This is very useful because it tags the rest of the team for a brief period so you can see where they are.

A warning though. If there is no one left on the team to revive the other players the team will be eliminated and you will lose your chance to shakedown the player.