Why is my finger tingling when I lean on wrist while typing?

Pressure on a nerve. Can alter the blood supply temporally causing tingling and numbness, partial pressure on one select portion of a nerve in your wrist can affect one finger, or multiple depending on the pressure pattern.
Sensory nerve. Pinching at the wrist. One of the most common symptoms of early carpal tunnel syndrome.

Related Questions

Finger tingling with some wrist pain. Is that cts?

Not necessarily. A pinched nerve in your neck, shoulder, elbow or wrist/hand can be a cause of tingling or wrist pain. Go see your doc for proper diagnosis. Read more...
Could be. Could be see you physician, after examination will refer you to a neurologist, for further tests , like conduction studies. Read more...