I think what is being stated is, you have a whip, hammer combo. So the whip goes first, in the combat turn, normaly the charger would now go after the whip, but due to being knocked down by the whip you skip the charters combat and now would make your hammer strike. Still having to hit him due to it being the same round of combat. It's not a new round of combat, so you don't get to auto kill him.
It's no diffrent then say your whip attack missed, the charger misses his attacks, and then you go with your hammer taking the charger out of action. You still roll to hit roll to injure.
I think the confusing part is, you get that first knock down/stun result on the whip. The hammer all its doing is giving you a chance of rolling a second injury dice. You still have to hit, wound, then roll a new wound result. So its like wounding with 2 swords. You get two wound results, take the highest. The hammer just comes after the whip.