"... If you’re curious to see how much faster ASP.NET requests execute without the thread switch, you can set the value to 0. This will cause the request to execute on the IIS I/O thread, without switching to a CLR Threadpool thread. ..."

Of course you would only ever set this if you are really doing every bit of I/O asynchronously in your application. Otherwise bad, bad things would happen (deadlock under load).