I don't know the answer to this ... what is your objective in wanting to swish? To clean out your mouth?
If that is all, then a quick rinse with Miracle-Mineral-Supplement in your mouth, then spitting it out, should be fine. I would rinse again with water a couple of times, to clear any residue.
I definitely would not do the 20 min thing, like with oil pulling, especially with Amalgams in there.