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.
Yes, Peyton List's character, Emma Ross, experiences a handgag scene in the Disney Channel show "Jessie." This occurs during a moment of comedic tension in the series, which often features humorous situations involving the characters. The scene is meant to be lighthearted and fits the show's overall tone.