A sex position in which the male hangs upside down by his ankles by a rubber cord attached to the ceiling. The female hangs on with her hands to whatever it is the male is hanging from, and this produces a nice bouncing effect with the rubber cord.
The term comes from the rubber cord used to attach the male's ankles to the ceiling with, and the term "mallard" is due to the fact that the male, when hanging upside down, resembles how a mallard looks when it is being hung upside down after being hunted.
The term is also the inspiration for a famous Ultimate Frisbee team, the "Rubber Mallards"
"What did you do last night?"
"Me and my girlfriend tried out the Rubber Mallard; it was amazing!"