It's a DirectShow filter which is loaded in place of the default directsound audio renderer.
It provides a new reference clock that is locked to the video card hardware clock, in order to ensure that frames are played at the exact speed of what is expected by the video card vertical sync.
It also provides a frame rate adaptator for media files that do not match a multiple of the video card refresh rate (ex: playback of 23,976fps IVTC NTSC on a PAL TV).
The combination of the two will give you the true experience of smooth playback with your PC.
Finally it is an audio renderer with hardware or software rate adaptation in real-time, multi-channel audio, audio timestretching (pal speedup compensation) and dynamic range compression capabilities.
does this mean that the video card hardware clock is changed according to the audio/video stream?