Charlie has written often about “paralysis by analysis,” and in my mind, concentrating on keeping knees up WHILE SPRINTING is an error.
But teaching proper technique through DRILLS is another matter. If you do high-knees, B skips, quick leg and other drills correctly and over a long enough period of time, better technique while sprinting will follow more or less naturally and without the athlete concentrating on technique which likely makes him slower simply because of thinking about it.