While sneaking, if you attack a critter(whether it's Player or NPC) from behind, you will always cause a critical hit with a +10 bonus on the critical power roll. Silent Death is that kind of perk.
|| Level 15, Sneak 125%
It's also component of speedboost.
Note: Silent Death does not affect grenades and only works for players that are either unarmed or equipped with a one-handed weapon with single-shot mode (SMG does not count, only pistols).