The Enhance Timing feature in GarageBand 2 works best on tracks that contain single parts. Because a typical drum loop contains several different rhythms across multiple sounds, such as the hi hat, kick drum, and snare, each drum part may have a different rhythmic bias that will affect the behavior of Enhance Timing. In some cases this makes it impossible for Enhance Timing to give useful results.
In contrast to the MIDI quantize feature, Enhance Timing depends on the content of the file. Any audio file that contains mixed rhythms may present issues for Enhance Timing. Drum loops are the most common example.