They told me.
One kid was waiting to hit, and knew the two on the tee box. He told me both of them could have carried the brook which is about 320 to hit the far bank and bounce out.
Of course, he may have been just saying so, except he hit a 3W almost to the front edge. Chose not to drive for fear of getting in the brook. He hit about 290.
So they can hit it that far. I am asking how does that relate to 80%. How did the kids know they were swinging 80%. I could try to take a guess at swinging 80%, it would probably be WAY off. Heck, Erik told me to do half swings, guess what they turned out to be, full swings. Just saying when someone says, "Oh I hit that 80%". Unless they actually see numbers, they can't prove it. Did they swing easier, they might have felt it, but the clubhead speed could have been faster. Jack gave a tip were when he wanted to hit it far he would try to swing with no tension and make a smooth backswing. Did it actually happen, I don't know.