When you are laying in a bath and you poke your bellend up through the waters surface, creating a small 'island' of bell.

You then proceed to rub the base of your penis slowly so as to not to create waves, and shout "Thunderbirds are go!" as you ejaculated.

A person with an outwardly dominant, masculine, or aggressive personality, but who also enthusiastically takes on the submissive or "bottom" role in the bedroom.