When the stylus hits a big transient the belt stretches and the platter slows for some brief instant. Once the transient peak has passed the belt rebounds toward its original (shorter) length, speeding the platter back up again.
I'm just wondering if anyone has ever tried to see this while playing an LP. You would have to put a speed calibration disc on top of an LP and play a song near the outside edge of the LP. If belt stretch was audible I would think that a speed strobe would reveal it. Anyone?