Two things worth looking at here.

New Zealand throw into a line-out. Keven Mealamu throws in. Standing at the front for South Africa is François Hougaard. The ball strikes his shoulder and bounces towards the New Zealand side.

Quickly the referee says:  "No knock. Off his shoulder."

Off the head or the shoulder is not a knock-on.

New Zealand form a tackle/ruck.

The referee calls: "Number 5, get back, 5. Behind the last feet."

The player he is talking to is Tom Donnelly of New Zealand. The requirement to be onside in a case such as this applies to both sides, not just to the defending team.