Where does the saying pounded like a mallard duck mean... Are mallard ducks pounded?

Okay so I had to google this after hearing it on South Park because I thought I was missing something. Since google has nothing on the phrase I'll presume it's just taken at face value.

What I mean by that is having lived near ducks I can vouch for their... unsettling sexual practices. Suffice to say that rape is a completely valid means of reproduction on a completely Darwinistic level.

So I believe getting pounded like a mallard duck would mean sex involving hair pulling and choking and such.