Backstabs

Baskstabs occur when a contestant is hit from behind with a melee weapon. They cause the attack to deal 50% extra damage. This can however be countered by the perk immunity, which causes the extra damage dealt by backstabs and headshots to not deal any extra damage.

(Note: the extra damage caused by backstabs can be increased to 100% extra damage by the perks Sneaky Poker and Brutus.)