In the original BMW, Peter Green refers to "magic stick", which is another word for the male reproductive organ. I haven't heard the Santana version in a while, but I swear Greg Rolie sings "break out my magic sticks", as if he (Santana) didn't understand the original reference. Apparently Sandra Elsdon who was Green's girlfriend at the time, was a part of a hippie-esque cult that practiced abstinence. It doesn't take much to wonder where "Rattlesnake Shake" came from.
Of course Peter directs the song at poor Mick Fleetwood.