.... The server also periodically polls the clients to ask for their current position in the file being played. If necessary, it commands them to momentarily speed up or slow down their playback in order to synchronize with the server's current file position.
The DSP plugin is used by the clients to synch themselves with the server. When a speed-up is needed, the audio waveform is horizontally compressed, and when a slow-down is needed, it's expanded.