The alphabet trick is an oral sex technique, particularly used in cunnilingus, wherein the person performing the act uses the tip of their tongue to trace the shape of the letters of the alphabet on the recipeint's clitoris. I suppose this could be used, as well, on the penis of a male oral sex recipient, but it seems highly doubtful that this would be an especially pleasurable variation compared to the usual, more engrossing activities. The breakdown of the technique is that it makes for a stimulating combination of circular and up and down or corner-to-corner strokes.

Let's consider this more directly. The alphabet goes:
Since very small variations can't really be felt all that much, translated into the strokes the recipient would end up feeling, this would be, roughly:
o | | o o o | o | - o | | o | | | \ / | | o o | o o | o o | | / o o | - o | \ / \ / \ / \ / \ / - / -
Whether this particular formulation is of any special consequence is an open question -- the real benefit behind the technique simply seems to be that it reminds clueless guys to alternate between circular and sweeping motions. If you already know to do this, it's pretty much a way to pass the time while performing if you're really not that into it; or possibly a distraction to keep from climaxing too soon if you're too into it. However, if you plan to be doing lots of oral on the ladies, it's probably better not to lean on this technique, and to instead just train yourself to alternate between circular and sweeping moments. That frees your thought process to focus on counting strokes so you can remember to occasionally remember to lick parts other than just the tip, and to gauge the girl's actual reaction so as to adjust accordingly to her signals of especially preferred motions.

On two occasions in the webcomic Questionable Content, character Faye has bragged of discovering that a boyfriend was using the trick on her.**