A reflex hammer is used to check how well your nerves react to the stimulus of being tapped. Your doctor knows where the nerve is closest to the skin in the elbow and knee. By taking the point of the reflex hammer and tapping it, your arm or leg should react in a certain way.
If the doctor taps lightly, he or she will not expect a wild or intense reaction but some reaction is expected. Likewise, if the tap is harder, the reaction would be expected to be a bit more intense.
Sometimes, it can be hard for the reflex test to be done if the patient is concentrating on it, so it wouldn't be unusual for the doctor to find a way to distract the patient. The best result is done if the patient isn't paying attention to the tapping.