That's pretty much it. That particular menu item has inline JS code that pops up the "Are you sure" dialog. When you confirm, the JS sends the Ajax request to the connector:
MODx.msg.confirm({
title: _('flush_sessions')
,text: _('flush_sessions_confirm')
,url: MODx.config.connectors_url+'security/flush.php'
,params: {
action: 'flush'
}
The flush.php connector sends the action on to the request handler:
$modx->request->handleRequest(array('location' => 'security','action' => 'flush'));
The request handler loads the processor (processors/security/flush.class.php) and calls its process() method, where the actual flushing occurs.
As far as I can tell, nobody along the line invokes any events.