Didn't know that was a thing. I'm guessing they just take whiskey, heat it up enough to boil off the alcohol but not enough to boil off the water, and then they're done?
I'd guess that they burn it off. It's fairly easy, though you can't get 100% of the alcohol out this way at home. You can make a vinegar out of liquor this way by burning the alcohol off of a handle of liquor, pouring a fifth back in, and adding a vinegar mother with some live vinegar to kick it off before waiting 4-6 weeks.