When a media begins, it sends a begin event. If another media waits for this event, it catches it. To make a media wait to an event, one of its synchronisation attributes (begin or end) should be written as follows:
<!-- if you want tag to start when
another tag begins -->
<tag begin="id(specifiedId)(begin)" />
<!-- if you want tag to start 3s after
another tag begins -->
<tag begin="id(specifiedId)(3s)" />
<!-- if you want tag to start when
another tag ends -->
<tag begin="id(specifiedId)(end)" />
For example:
<body>
<par>
<img src="cardiff.gif" region="cardiff"
id="cf" begin="4s" />
<img src="next.gif" region="next"
begin="id(cf)(2s)" />
</par>
</body>
will make the next.gif image begin 2s after cardiff.gif begins.