Can confirm that the you do indeed kick down on the pedal that is shaking. It is designed to prevent excessive build-up of side-slip which can bite you seriously in the Harrier if coupled with high power and moderate AoA in the 30-120kt flight regime; i.e. decelerating into / accelerating out of the hover. Google Intake Momentum Drag. In the Harrier II the pedal shaker is triggered at 0.06g (lateral acceleration). Believe John Farley can explain in great detail why this particular number was chosen but this 'g' equates to around 1/2 aileron input to maintain wings level with the gear down (zero-scarf nozzle) at typical approach speeds, giving you a margin to correct any potential problems. Side-slip + AoA + High Power (intake air momentum) = departure and it's killed a fair few unwary chaps in the early days before it was really understood. Nowadays the modern Harrier is fitted with a Stability Augmentation System to 'smooth' down the ride....
Hope this helps...