Brink

There are two ways to obtain this trophy, but most likely you will get it before getting the ability to get it the second way. You will know when you are down, because you will have a little image in the middle of your screen with a down person laying on his side.
 

1. Most of the time enemies will sprint towards you and slide to hit you knocking you on the ground, you are still alive and can still shoot your primary weapon, all you have to do is shoot at the enemy and kill him before getting back up or before the enemy comes and finishes you off.
2. The second way is, when you reach rank 5 you can use one of your level Credits to purchase the Downed Fire ability, that ability is under the first section of the abilities menu, marked under Universal. You can access that menu from the main menu or once you have completed a mission or failed, it will pop up in the end if you unlock a new ability from leveling up.