You may be referring to a test called Tinel's test. The doctor uses his or her hand or reflex hammer to gently tap the ulnar nerve as it passes close to the skin along the inside of the elbow. If the test causes pain, numbness, or tingling down the arm, then the test is considered positive for ulnar nerve entrapment. Another name for this problem is cubital tunnel syndrome (CuTS).
This test by itself isn't considered reliable. In at least one-third of all normal adults tested, Tinel's test was positive when there was nothing wrong. This is called a false positive test.
The doctor must rely on a series of tests to confirm or rule out CuTS. This may include the patient's history, clinical exam, and special tests. Tinel's test will be one as well as the elbow flexion test and perhaps even a test of electrical impulses along the nerve.