The default link text is Expand / Collapse. This can be changed by configuring the extension.
For these examples the extension is configured to use the link text: Custom Linktext.
Given the spec reads like this:
<div id="id" class="collapsible">This should be collapsible.</div>
When run the output is enhanced with the needed code for the collapse feature:
<div class="toggleContainer"> <a href="javascript:Collapsible.collapseExpand('id');">Custom Linktext</a> <div id="id" class="collapsible" collapsed-height="1.5" collapsed-height-unit="em" style="height: 1.5em;">This should be collapsible.</div> </div>