The simple explanation...

Once you have configured your browser to use Immunicity, when and only when you attempt to access a website that is typically censored, your browser will route your request via our servers. Any other request, for example to Google or Facebook, will go directly to the destination without passing through our servers.

You can see exactly what sites will pass through our servers at immunicity.org/blockedsites.

The technical explanation...

When you configure your browser to use our service, you are entering the location of a PAC file ("proxy auto-config"). This file contains the latest list of URLs that are commonly censored in the UK. When your send a request to one of these sites, your browser is instructed to route that traffic via our HTTP proxy server using a uniquely generated address which will fetch the content and send it back to you. When you send a request for a website that isn't on our list of URLs that are commonly censored, your browser connects directly to that site, bypassing our servers completely.

We recommend at this moment in time that you don't configure your browser to explicity use our proxy manually without using the PAC because we actively deny access to sites that aren't on the censored list to prevent abuse. You can however save a local copy of our PAC file and use that, but remember that we do often update it, so if you do save it, update it regularly. If you already need a proxy to connect to the internet, then Immunicity probably wont work.

This is what a normal request to The Pirate Bay commonly looks like.

This is what a request to The Pirate Bay looks like through Immunicity - fully working.