Andrew,
If you quickly push the prop control in (high RPM, fine pitch), you can indeed overspeed the engine, but you will definitely have a temporary speed brake as energy from the aircraft's velocity is used to increase the propeller RPM (or decreased thrust is available depending on the flight condition; it feels pretty much the same from a practical standpoint).
If you quickly pull the prop control (low RPM, coarse pitch), the energy stored in the rotation of the prop will impart extra thrust to the airplane, giving you transient increase in the airspeed.
Once the RPM stabilizes, the transients have no more effect and the airplane will respond to the new power setting (usually lower power and hence decreased airspeed with lower RPM, higher power and resulting increased airspeed with higher RPM).
The effect is dependant on the change in the RPM and the mass of the prop. The transients are very notable with a heavy prop and large RPM change.
Pat
