I'm a bit surprised no one had yet answered this -- or for that matter, that it was asked. A "handgag" is just what it says: Gagging someone by clamping one's hand over the person's mouth.
A handgag is a technique used in self-defense or combat situations where an individual covers another person's mouth and nose with their hand to muffle their screams or calls for help. It is intended to immobilize the person being handgagged and prevent them from making loud noises.
The handgag is used to describe a choking technique. Typically, the attacker comes from behind, covering the victims mouth with a hand, to restrict the flow of air.
Handgag is simply when one person puts their hand on another persons mouth to stop them from speaking or screaming.