What 'dynamic' means is that the plugin would let you set min and max length for a crossfade vs a static value. For example as opposed to just setting a value of 4 seconds you could specify a max of 8000ms and a min of 50ms, then depending when the track fell below a threshold during the end it would decide if it should start the next song right away and let the last fade out underneath, or if should wait until the very last moment, providing a sort of gapless playback.
It was very musical for disparate tracks and proved to never be jarring as we've all experienced with 'traditional' crossfade.
