I have taken this test a couple/few months ago. The procedure went like this for me:
Robe only on top.
Stand on a platform w/ (basically an xray[MRI?]) sensor over your chest.
I started the "action" by taking a mouth full of bicarbonate crystals chased by a cup of water. (you'll want to burp from all the gas created, but try not to - per instructions)
Next, you get some wonderful chalky thick fluid to drink. (Mine was nicely chilled and did not taste horrible).
The doctor starts the picture taking and tells you to swallow, swallow, swallow, fast and keep on till he says stop. (you'll probably be able to watch the screen to, really neat and wild seeing it happen)
Then the platform leans all the way back, flat, with the scanner still over you. So you're flat on your back and you do some swallow and picture taking in that position.
While laying flat, the doc will have you go left side-swallow-picture and right side-swallow-picture.
Doc will probably also be poking and prodding you're stomach (I think it's to see if the barium pushes back up the esophagus, maybe to see how the lower sphincter holds up with pressure?)
Then you'll do the swallow-picture routine laying on your stomach.
By this time, you're pretty bloated and sick of swallow that stuff.
That's about it. Oh, you'll poo white sometime in the next day or so, really weird!
My doc said the swallow test did not show any issues.
On a side note, my daughter (3yr old), whom has several issues, had the barium swallow test done a month or so ago, and you could really see the reflux happening as she was swallowing the stuff. So I've seen both and you'll really see it if it happens.
Also, I've had the endocsope and this barium swallow, been on nexium, zantac, and recently added carafate. So after all this at my last doctor visit, I asked him bluntly, "So doc, I've had all these tests and found nothing wrong. Been on the meds for awhile. What's wrong with me, how long will I have to be on the drugs, and will I ever get better?"
He told me basically and also bluntly, like fibromyalgia, we sometimes don't know why gerd starts, what causes the complaints, how to cure it, or how to make it stop, where the weird pains come from or why. It wasn't a helpful answer coming from my doctor, but it was honest and what I suspected.
Anyhow, now that I've gabbed on for more than I intended, back to our regularly schedule programming.
-Noggin