Self destruct

Self destruct is one of the six perks available at level 15. It is commonly used for trolling and getting multi-kills. When a player uses self destruct, he will blink for about 1.5 seconds before he explodes. When he blinks, he is indestructible and can't be killed. The explosion can only harm players of hostile teams, it won't do any damage to walls or turrets. It's a very costly (it costs your life) but powerful perk.

Using
The best way to use self destruct (also called SD by other players in the game) is to find a large group of enemy players (mostly in the middle) and just press space and kill them all, along with you. It's only recommended to do this when you are cornered and know that you cannot escape alive.