This plugin has nothing to do with game callback. Its a website callback. Once installed see the useage help page: your-site.com/callback-guide/ or your-site.com/?a=callback
This flow is like
1) If website owner provided callback url in his account settings e.g his-site.com/callback.php
2) when user votes ( via modified voting url, again see the useage help page ), we send a request to the owners provided page above with certain values so he can identify the voter on his website
3) if he integrated his callback correctly, he can check on his website if a user voted or not and reward him some stuff