Wanking like a duck is a common pun involving the resemblance between the pronunciation of the word "wank" and the cry of a duck.
It means that someone often speaks with a nasal voice.
- Where's my cow? (in a redneck manner)
- Hey man, stop wanking like a duck!
A spirit duck is crushed when a person passes gas (farts, toots, etc). It quacks but the duck itself cannot be seen because it's a spirit duck. An important distinction from "stepped on a duck"
(Person makes a loud fart). "Oops, I just stepped on a spiritduck."