New guy here, but finally I can speak on a topic I actually know something about. If it weren't for the P-lip I would not smoke a pipe. A fishtail directs smoke directly onto the edge of the tongue, and for me, despite many different pipes, tobaccos, and filling techniques, always resulted in tongue bite. The P-lip has the hole directed up and back, at maybe a 45 degree angle, so the smoke goes above the tongue and hits the palate. As it courses through the mouth it must cool considerably, because I never get "palate bite" from my P-lips. Only drawback? - the bend adds resistance to draw, so some P-lips ( I have a 999 rhodesian I am tying to work on right now) have too tight a draw and feel like sucking a straw in a milkshake, but of the 10 or so P-lips I have, this is the only one that has this problem.
Hope this helps,
Paul