We know this- but there are many ways to know if shaft flex is being held.
My students can do it whilst doing drill 1- here is a student doing the left hand drill- as can I doing the two hand drill
If we increase torso rotation speed and hand speed (from drill 1) against the system that is trying to slow it down it can be done.
I have been able to do it all my life because I have never tried to hit the ball with the clubhead- I have tried to make the ball move by using the entire club particularly the grip end.
And we can also hear it by the extra sound a sprung shaft puts on the ball like these from 93 Masters
{vimeo}21418033{/vimeo}
People will always argue about photos and assume you just need all the fancy tech stuff to show something.
But John and I know we can see ground forces by doing drill 2 with the carpet (for eg) Just like if we hit PV5 with a vertical shaft we all know the pivot is moving faster than the club or hands can.
And all these have a bearing on the shaft flex.
I had one of my students be filmed by the Sportsbox people and they called me up that evening to say they had never filmed any other player who did what this player did. And that was his hand speed whilst trying to slow down due to the radial club release actually picked up speed beyond impact again. Not just picked up speed but went back to the highest speed the hands had throughout the entire swing when they maxed out. Out of hundreds of players they filmed for their research only my guy did it different. But of course his data will get thrown away because it isn’t the norm and because everyone else does it another way they will say that is the commonality.
Here is a normal golfer on the left who was showing in a video what “should” happen…with continual hand stall so the clubhead swings by and picks up speed (but does it?)….compared to my pro on the right who picks up hand speed and therefore is continuing to stress the shaft because the hands are now increasing in speed against the system trying to slow them. Only guy out of hundred they have measured 
