That game was far too close for comfort and saints did very well to win it.
HKR used a tactic to stop Saints quick PTB, stop off loads and put players in a position to knock on. It happened at least twice to Graham.
The second tackler looked t clamp the ball an get his hand/arm in the gap between the ball and the upper arm. It made the saints players focus on keeping hold of the ball rather than doing something with it or getting on their knees/front for a quick PTB.

Clever and, I think perfectly legal.