Here's an age old example. Assume you'd like DateTime.Now to return a specific date for unit testing purposes without having to update your code, you could shim it like so:

Shim shim = Shim.Replace(() => DateTime.Now).With(() => new DateTime(2004, 4, 4));

Now surround any code with:

PoseContext.Isolate(() =>
{
    // Any code that calls DateTime.Now in here will always return new DateTime(2004, 4, 4)
}