It involves two females "69ing" (performing oral sex upon each other) whilst the male inserts his penis into:
a: The vagina woman on the "bottom" leaving the woman on top to stimulate both penis shaft and clitoris of her partners.
b: The vagina of the woman on "top". This is often the most pleasurable as the woman on the bottom can now suck the testicles of the man.
Variants are of course commonplace such as inserting the penis into a willing mouth or even a rectum. However such acts divert from the true 70ing experience.