Sometimes you will get knocked down but actually still be alive. Before you get up, kill an enemy. You get knocked down by melee attacks (not knife), explosives/grenades, or even by someone jumping down and landing on you (usually accidently).

Note: There is an ability called "Downed Fire" which lets you shoot while incapacitated. You must be Rank 5 to unlock it, but this ability is not required for the achievement, even though the achievement picture is the same as the ability picture