This effect allows you to change the pitch and the speed of a sound independently. It is very useful for example to constrain a recording to a certain length. The algorithm used is Appleās AUPitch Audio Unit, which provides professional quality even though it is included with every copy of Mac OS X.
Increasing the quality results in a deterioration of processing speed and vice-versa. The adjustment of the pitch is done in semitones, i.e. +12 corresponds to increasing the pitch by one octave and -12 to decreasing it by one octave. If lock is selected, the speed increase/decrease exactly matches the pitch increase/decrease in such a way that the result is the same as what would be obtained by just playing the sound back at a different speed. In that case, the sound is simply resampled and does not go through the AUPitch effect.