If a finger can only be straightened after being bent following a painful "snap", this is what's called trigger finger. The cause is a thickening of the flexor tendon, a tendon node, which prevents the tendon from sliding past under a retaining ligament in the palm.
Initial symptoms occur when a finger does not close spontaneously when the patient makes a fist or only straightens again after a nudge. Pain later develops when the finger is straightened. In very late stages of the condition, the finger has to be passively straightened by the patient himself.
In a small surgical procedure, usually carried out as an outpatient and under local anaesthetics, the ring ligament preventing the tendons from sliding in the palm of the hand is separated. The finger then immediately regains its mobility.