From the manual:
"An allpass filter will pass all frequencies, but will have two effects on a sound. The first effect is that all partials are shifted in phase, higher partials are shifted more as lower partials (there's a detailed explanation of this in the SOS-series). This phase shifting effect is not heard unless the output is mixed with other (allpass filtered) sounds of the same basic pitch. The second effect is that when the resonance is opened a strong resonant peak occurs at the cutoff frequency. And this effect can be put to very good use when e.g. filtering audio samples. The peak can significantly boost a small frequency band in the sample, without filtering away the high or the low."

Here's an example for the first effect:



Allpassfilter2.pch2

And here's the resonant peak:



Allpassfilter.pch2