In Western films it is not unusual that when the trigger is pulled against the hero, a fire does not follow, and the crowd cheers that part of the mechanics inside the firearm got stuck. This is similar to what happens with trigger finger: one’s finger is stuck bending and cannot be smoothly straightened.
When we move our finger, the muscle pulls the bone via the tendon, a fibrous cord connecting the muscle to the bone. While we move our finger numerous times a day, we don’t have to worry the wear and tear of the tendon, because it is surrounded by a sheath and protected from abrasion. However, if the sheath narrows down by inflammation or other tissue changes, the tendon can be trapped so that we cannot pull the trigger, our finger, freely.