Burke might have seen something, but I'm pretty sure John Mark Karr didn't.I find it interesting that this description by Burke is almost identical to John Mark Karr's description of what took place but with Karr being the one weilding the blow to her head. Even to the point of Burke saying JB had been carried "quietly" downstairs by "someone".
It sounds as though Burke might have actually seen certain aspects of the crime, depending on whose description is factual. And possibly can be why LE keeps trying to question Burke in more detail.
The way Burke showed JonBenet being hit on the head is the obvious way. If you asked me to pretend to hit someone on the head, that's the way I'd do it. Now if JonBenet was struck with an upper-cut and Burke had pretended to do that, I'd find it more interesting. But I'd still try to eliminate other ways he could have gotten that information--family, attorneys, news media. Ditto JonBenet being carried downstairs.