Webmentions plugin
This plugin enables you to automatically send Webmentions to all URLs mentioned in your post every time you publish it in your collection.
Note: This plugin is for sending Webmentions, not for receiving them. See here for receiving Webmentions: Feedback Options.
When do the pings happen?
The pings are sent out when "notifications for a new post" are sent out. At this time, several "ping" plugins may get triggered.
For additional information on what happens after each new post, please see: After each new post
Location
The Webmentions plugin gets shipped with b2evolution and can be found in /plugins/webmention_plugin/_webmention.plugin.php.