A sexual technique
mentioned, but never explained, in an episode of LA Law
The alt.sex FAQ, such as it once was, mentioned one possibility: the giver puts their hands together and spreads their fingers apart in pairs. One pair stimulates the clitoris, one or two pairs penetrate the vagina, and one pair stimulates the ass of the receiver. A nice triple play if you can manage it.
Since there is no accepted definition of a Venus Butterfly, plenty of people will be glad to sell you videos on how to perform this "lovemaking secret". There are also vibrators named "Venus Butterfly".
The bottom line is that nobody knows what the Venus Butterfly really is, except perhaps Steven Bochco, and he's not telling.
You can, however, have lots of fun trying to find out what it is.