If you notice, the guy in the Utube video is smarter than the average bear... he hits the manifold with a laser temperature gauge when he's putting the fire to it...
Don't believe for a second you can't melt the manifold with STRAIGHT AIR. If you don't have a laser temperature gauge, the other way you can tell the manifold is getting too hod is when you smell a sweet odor. This smell is the only other sign that I know you can tell the manifold is ABOUT TO MELT!
Otherwise, when in doubt, stop and let it cool.
We have always cleaned manifolds by first, clawing out as much as we can with a bent screwdriver. Then we pressure wash. Finally, burn out with an oxy-acetylene torch with an air nozzle. We can have it cleaned out and ready to go back on the vehicle in about 1/2 hr.
How often does it need to be done? My own A4 has 200,000+ miles and has NEVER been cleaned, nor does it need to be. Drive the engine right and it doesn't need to be cleaned. Some people think they need to run the engine in low rpm's to get great fuel economy. What they are doing is called Lugging, and it's hard on the engine. If the engine is run at the proper rpm range, the intake cleaning becomes less- to no-issue. You'll still get great fuel economy, without wrecking your engine.