We observed the dynamic interaction between a fingertip and an ultrasonically vibrating plate using Laser Doppler Vibrometry in order to investigate the causes of ultrasonic friction reduction. Observations were made both for a human finger and for artificial fingertips constructed to exhibit different amounts of damping. The data suggest that fingertip dynamics play an important role in friction reduction. In particular, the fingertips were all found to exhibit forced oscillations in response to the plate motion, but with different relative phases. Fingertips with lower damping oscillated more in-phase with the plate, while fingertips with higher damping oscillated more out-of-phase with the plate, and also exhibited greater friction reduction. These results are reflected in a model.