It wasn't 6 again set restarts in own 40, its the wave of the arm to signify a set restart after a knock on. Was at the game also and explained it to the bloke next to me who questioned it. It happened twice after contested kicks.
No bell went off, which means its a set restart for a knock on or handover. Todd maybe waves his arm around the same for either call, but he wasn't giving 6 agains instead of penalties, he was giving neither. Just letting players know it was tackle 0 after the knock on.