they only tighten during impact. I am saying that good players tighten their grip radically during a short period around impact, whereas average players tighten their grips over a much longer period.
Anyway to me biggest benefit with Mindy's technique is not distance but consistency and accuracy. He wasn't saying you would become long distance driving champion with this technique. I think he was trying to demonstrate that by driving with the legs you get an accurate repeatable golf swing without sacrificing too much distance.