Everytime a player kills someone they get a bonus of (Default 20 HP) added to their current HP. This is helpful especially for those big servers where its impossible to kill more than several people before dying.
CVARs
amx_kill_bonus <hp> - Defines the amount of hp added to the player every kill. (Default = 20)
amx_kb_limit <hp limit> - Defines the max. hp a player can get. (Default = 100)
Threaded Mode