Here is a video shot at the 18u trials in Cary last week for the USA Baseball with Bryce Harper throwing runner out from his knees.
It was the first time I saw him play and it looked like he was setting a runner up for this in the prior inning but the batter struck out for the 3rd out. Its slow mo but in real time its really fast.
http://www.youtube.com/watch?v=YDDgYlvLTWE

nice!

The kid is pretty impressive, but I do have to wonder about that set-up with a man on base. He's got his left leg extended out so he can quickly "step" with it in order to throw down without the usual footwork.

But what happens if the pitch is in the dirt to his left or right? From that position, it seems as if it would be very hard to move laterally to block a pitch.

How'd he hit there? Hasn't done well at the plate lately from what I've been hearing.

Here is a link to the USA Baseball stats page.
http://web.usabaseball.com/teams/index.jsp?team=2494
He went 1-4 the night we saw him. Swing looked good.
I recall he was one of the higher average guys for regular players for the 18u team.
I think I have a slowmotion swing that I will try and post if I can find it.