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I went to the MLB highlights for the pirates from 2007 and looked at two good games from June. He consistently locked his knee and rotated around the knee from 3rd to 1st with the toe pointed roughly midway between 3rd and home. In short, his past success was with the locked front knee.
I rewatched the video you have linked above. He looks more relaxed on the follow through for most of the pitches, lest torque and more drive. In fact it's quite surprising how short his stride is for the power he generates. Additionally, he looks like a good athlete on the mound. With a flexed knee he should be able to field well from the mound.