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.
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.