press, pull, squeeze
tighten on
Her finger tightened on the trigger as she heard footsteps approaching.

finger
guard
mechanism

have your finger on the trigger (= to be ready to shoot)

Trigger is used with these nouns as the subject:
infection, stress

Trigger is used with these nouns as the object:
alarm, alert, allergy, association, attack, avalanche, backlash, change, collapse, controversy, crisis, debate, depression, disturbance, earthquake, emotion, eruption, explosion, fall, fit, landslide, mechanism, memory, migraine, reaction, reappraisal, recession, reflex, relapse, response, retaliation, riot, search, spiral, unrest, wave