Breathe out, and tense your abdominal muscles suddenly so the breath stops sharply. This should pressurize your throat. Hold the feeling, and if you've done it right, you won't be able to breathe in through your nose or mouth until you let the pressure relax. It should be obvious how to relax it but in case you're worrying, simply breathing out will do the trick.
Think of the way American football players shout "Hut!" (or whatever it is, but it sounds like that), and try to make the same noise, if it helps.
Once you've done this, hopefully you should be able to repeat the feeling more naturally.

Hope that's a good way of describing it? I've always been able to do it naturally but that's how someone once described it to me (as a singing technique).