Strangulation protects you from drowning, since you cannot inhale water if you can’t inhale at all. It also prevents potion vapor inhalation effects and the inhalation effects of gas clouds. Basically it works like magical breathing except you die in 6 turns.

You can apply an amulet of strangulation to an adjacent monster (that has a head and neck) which will strangle it in 5 turns. Possibly only if the monster is sleeping or paralyzed.