Knock out

during gameplay Snake can disable his opponents by knocking them out with particular weapons or CQC.

Regular gameplay
Using tranquillizer weapons enemies can be put to sleep. Often they will require multiple hits and the weapons have to be reloaded manually. Because of this there is a possibility of them spotting you before they fall asleep. Opponents can be stunned using stun grenades, using a choke or by hitting them enough times. In both situations given enough time they will wake up and cause an alert. If they are seen by another soldier they will kick them until they wake up. In Portable Ops recruiting an opponent requires you to knock them out before getting them back to the truck.

Stamina kills
During boss battles the life gauge of your opponent is displayed on screen. using stun techniques the stamina gauge of your opponent can be reduced. Once it runs out they are knocked out. By using this method you will often receive special items for not "killing" the Boss.