I found that last year and liked it a lot....but it always makes me want to correct people on the upper lip thing...
If we look back to the original sutta at the word "mukha", it literally and sensibly means "entrance" or "mouth." If we give it this obvious meaning we have: "He fixes his attention at the entrance'", the entrance being either the nose or the mouth. The early commentators are assuming the reader realizes that the meditator may be breathing either through his mouth or his nose. If he is breathing through his mouth he should direct his attention to air contact at the lip. It is very sensible advice really, for it would be a shame to have to give up breath meditation just because one has a cold or a plugged nose!
More good stuff there on nimitta...
Diverse written works by modern teachers mention the characteristics of sign and counter-part sign appearing during breath meditation. Often these descriptions take the classic simile description found in the Visuddhimagga, perhaps as a cautious attempt at not straying from orthodoxy. However, as we show below, this description of the sign (learning or counter-part) may turn out to be quite misleading and, as often expressed by frustrated meditators, unclear.