A Windows Phone senses how you're holding it and flips between landscape and portrait accordingly.
Occasionally it would be useful to override this and fix the phone in landscape (or portrait) regardless of how it's held.
Is this possible?
Technically speaking you can't. Some apps can implement the behavior they want but you can choose one or another.
There is an homebrew app that can do that but you will probably lose the warranty.
I've noticed that it does not orient when it is upside down.
You can take advantage of that in some cases, flip it upside down and it retains the previous orientation, e.g. landscape even when being held portrait-style. (Helps a lot when reading in bed laying on my side...)