Basically this type of testing puts you into the code. I can't say I've done it in the past few years now, but what it strives to do is force a branch into code that you rarely get to see.
An example might be, a web landing page, for instance. Let's say you are planning to launch a login page for your website starting on the 15th of this month. So the script inside your webpage reads something like this (pseudocode):
<font class="small">Code:</font><hr /><pre>
If serverdate(now) > September 14th then
Redirect to new login page
Redirect to old login page
The only way we can test this right now is by turning that greater than operator around and forcing the redirect condition to be true.
So, basically, you can use this anywhere in your app that you aren't able to touch from the interface itself, with manual testing.
9 out of 10 people I prove wrong agree that I'm right. The other person is my wife.