It calculates a percentage which shows how many of the requests for this page, are being served by a Varnish Cache Proxy!
The Varnish Cache Efficiency Extension is useful, if you want to check how many of the request
necessary for the current page are delivered by a Varnish Cache Proxy. The shown number is a
percentage value which says how many percent of the requests are delivered out of the Varnish
To function the Varnish Cache Proxy needs to be returning the "X-Varnish" Header.
More features are yet to come!