You no longer have to use this plugin because it was merged to PPP plugin at v2.6.0.
This plugin send Cache-Control: no-cache, must-revalidate header when show public-post-preview post.
In other word, that make skip cache of CDN (like CloudFlare, CloudFront… and other), or Proxy-Cache (Like Nginx).
So, outside people always can see newest draft, and there is no need extra time to close public draft.
This plugin needs PHP 5.3+
Note: If you’re using CloudFront, you should also use C3 Cloudfront Cache Controller