"Pulling a Hartung" is an in-joke for fans of Scotch and Smoke Rings, essentially meaning to concede in a game of Hearthstone.
It is a reference to long-time fan, Federo Gregoro, often conceding to Oxhorn in the game when he knows he has lost. This is polar opposite to Ox who never concedes to an opponent.
"What?! Mal'Ganis?... I'm about ready to pull a Hartung here ladies and gentleman."
—Ox nearly pulls a Hartung of his own against Gentleman Badger.
"Oh! What? You've got Thaddius on the board and you'd pull a Hartung?!"
—Ox shocked at Greg Hartung's concession on Episode 308.
Oxhorn once nearly pulled a Hartung while doing battle with Mal'Ganis. He has otherwise never attempted to concede in Hearthstone.
The phrase has become so iconic that other fans sometimes refer to Oxhorn simply losing the match as "Pulling an Oxhorn"