pertaining to carabiners and snaps. requiring one motion for the gate to open. A nonlocking carabiner is single action.

single action (Wikipedia)
Trigger mechanism in a bolt action rifle

A trigger is a mechanism that actuates the firing sequence of a firearm, airgun, crossbow or speargun. A trigger may also start other non-shooting mechanisms such as a trap, a switch or a quick release. A small amount of energy applied to the trigger causes the release of much more energy.

In "double action" firearm designs, the trigger is also used to cock the firearm — and there are many designs where the trigger is used for a range of other functions. Although triggers usually consist of a lever actuated by the index finger, some such as the M2 Browning machine gun use the thumb, and others like the Springfield Armory M6 Scout use a "squeeze-bar trigger".

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