The tendons in the fingers that cause the fingers to bend (flex) may develop a nodule.
Finger flexor tendons travel in a slippy sheath along the finger, and the tendon slides in and out as the finger bends and straightens. The sheath lubricates the tendon and also prevents bowstringing of the tendons as the finger bends. In cases of trigger finger a nodule develops on the tendon. As the finger bends the nodule passes out of the sheath into the palm. As the finger straightens the nodule attempts to pass back into the sheath but becomes locked at its mouth. Preventing the finger from fully straightening. The finger may be forcibly straightened with a click but should be avoided.
Locking of the finger/ thumb. The fingers fully bend (flex) but on attempting to straighten they lock with a bend. Occasionally the symptoms are not as dramatic as this but the patient may notice clicking or even feel a nodule at the base of the finger/thumb.
A classic trigger finger/thumb is diagnostic. A nodule may be felt at the base of the finger/thumb.
Occasionally the finger/thumb is locked in a flexed position.
Here is a short video clip of one of my patient's demonstrating classic triggering.