It sounds like reverse sneezing, my Karlee does it too. It usually happens when she gets excited, or eats or drinks too fast. I know how worrisome it is, and if it turns out to be the same, you can learn certain techniques to help your pup relax the spasms in the throat. Reverse sneezing is fairly common, and is not harmful to your pup. It sure does scare you though, because it sounds like they are choking to death.
It's good that you have a vet appt., then you will know for sure. Dogs with kennel cough, or collapsing trachea can sound similar, but those are very different. Good you got a video of it in case you can't make it happen when at the vet's.
This thread has several videos and info about reverse sneezing:
http://www.yorkietalk.com/forums/sic...2-weezing.html
Good luck, and do update when you see the vet.