Yeah, moisture activated. Leaking refrigerant cools the area where the leak is located, causing condensation from the surrounding air. The condensation activates the epoxy in the sealer, plugging the leak. At least in theory.
Short exposure to atmospheric isn't a problem. I changed out the dryer in my car while at least some of this stuff was in there, so the system was exposed for perhaps an hour. It's been three months since then with no problems.
The sealer circulates with the oil in the system. So flushing the system removes the sealer.
It's not a panacea, but if you can't afford or can't find someone competent to tear apart the car replacing the evaporator, it's an alternative.
From an economic (and definately not environmental!) analysis, I think the sealer almost always wins. If the sealer slows the leak to where the car consumes, say, $50 of refrigerant per year, you will never recover the cost of replacing the evaporator over the remaining life of the car.