Clock slaves get assigned a clock by their containing pipeline. Their
task is to make sure that media playback follows the time progress as
represented by this clock as closely as possible. For most elements,
that will simply mean to wait until a certain time is reached before
playing back their current sample; this can be done with the function
gst_clock_id_wait (). Some elements may need to
support dropping samples too, however.
For more information on how to write elements that conform to this required behaviour, see the Plugin Writer's Guide.