B2B stands for body to body and as such is pretty self defining. It is a sexual act more than a traditional massage where both parties get naked and the masseuse rubs her body all over the male’s body.
At the start the man is often face down; she rubs his legs, buttocks, back, mainly with her breasts. It usually quickly leads to the “flip” when the male turns over on his back making his erection accessible. The masseuse then repeats the process starting at the legs, moving up, often stopping to suck on the erection, to massage the erection between her breasts and continuing upwards to allow the recipient’s mouth access to her breasts. Often she will then continue to the point of “massaging” the head/mouth with her vagina, the subject performing oral sex on her which in turn often leads to her turning around for what is referred to as a 69, with both of them performing mutual oral sex.
As you might surmise, at this point both are quite sexually stimulated and will often continue on to some form of intercourse (cowgirl being common) although some will simply finish everything off by providing a “happy ending” for the client, i.e. continued stimulation to the point of orgasm.