Boom boom pow means a girl is getting fucked by two guys at the same time (one in each hole) and the sound of the two guys fucking her makes the boom sounds and when the girl reaches orgasm she "pows"
This term is used in the black eyed peas song Boom Boom Pow and Fergie sings how she loves the boom boom pow.
"that girl only gets off with that boom boom pow"
"she is so horny she wants some boom boom pow"
"Fergie gets boom boom powed by the other black eyed peas"