[T.G]

Not bickering Peter, just trying to make Chris think about part of the answer a bit for himself before I spell it out.

The ammonia is present in the leaf, as you note, I've always said it is a component, and during combustion, it comes out at a rate proportional with that of the burn rate of the cigar. But it doesn't pocket or collect as you smoke the cigar. The most that could happen is that a small amount could be re-absorbed into the leaf as you draw through the cigar, but once absorbed, you won't be able to "blow it out of the cigar". If this were the case, you could take an under fermented ammonia laden cigar and blow through it a few times and there would magically be no ammonia.

Now, tar and some of the other impurities, on the other hand, are a semi-solid sludge that comes out as the leaf is heated, prior to combustion, and it doesn't really go away but does collect just forward of the heat source (you can't really see it on a tobacco leaf without magnification, but light one end of a 2x4 and observe the area just forward of the flame and you can get an approximation of what is going on). Thing is, it's pooling on a porous surface - so as moves up the leaf, it builds up and leaves deposits - by blowing back through the cigar and the application of flame to the foot, you are forcing this sludge into the fire to be burned off.