Posted for REHBINC@aol.com:

 Expanding ever so slightly on George's statement:
 
 During periods when you can't possibly make too much power, there really
 isn't a need for compressor speed control. However, much of the time you
either
 need, or want, to operate at lower power settings than the engine is capable
of,
 such as taxiing and low altitude flight. In such conditions you can use the
 throttle to reduce manifold pressure, or you can reduce the compressor
output,
 or both. Since it takes power to run the compressor, it is much more
efficient
 to reduce the compressor output. In periods where the limiting factor is
 manifold pressure or cylinder temperature, the reduction in compressor output
 diverts additional power to the propeller, so you climb faster while you save
fuel.
 
 Remeber that an engine is most efficient with the throttle wide open.
 
 Rob