The button looks like this:
The grey area will show the number of users that have spurled the page, and allows users to spurl the page simply by clicking it. If the users aren’t spurl.net users already, the pop-up will direct them to the spurl.net webpage for information and sign-up if they want.
So, the button helps promote your site through spurl.net and helps building spurl.net by drawing users and usage from already existing users.
To get the button, simply go here for instructions and the code sniplet.