I'd like to repeat an ol' question I had about DH, which is still unresolved and relevant for BC.
The Knock-Down action is an opposed Strength test. It says nothing about needing to actually hit whoever you are trying to Knock down first.
Yet I think the targets Agility and ability to evade should enter the picture here, otherwise some big, slow brute will routinely swipe the floor with the agile assassin. Knock-Down, Attack. Then the Assassin needs to Stand (Half Action) and might have taken damage and fatigue. This severely reduces his chances. Rinse, lather, and repeat.
Have I missed something? Can you Dodge Knock-Down? At least Grapple requires a WS test to Hit, that can be dodged.