You know what the problem is? The argument can be made that we didn't get to see every little thing that happened during the forest of death portion of the test. I have zero interest in going back and seeing if there was any wiggle room for that karin flashback scene to occur, but it's not beyond reason to assume it's possible if you choose not to check. So, assuming we did not see every little thing that happened in the forest of death part of the chuunin exam, it's possible that Sasuke had opportunity to test out Lee's move, which goes a long way to explain why he could pull it off so effectively during the preliminaries.