P54 GSotF "the compressed ball pushes itself off from the clubface, having first picked up the speed of the clubhead." p55 "It follows that, if you can keep the pressure up through the ball, you can have the clubhead travelling at 85 mph or, if you are strong enough, even 90 mph."
So in Blake's mind pressure was used to keep the clubhead moving through impact and stop it losing speed before the ball was released from the face.
From Kelley TGM p25 "the ball acquires.....100% of the Clubhead "separation" speed (so there must be resistance to deceleration)" and " Treat that "heavy" feel of "Clubhead recovery" after Impact as though it were all Impact, even though the ball is actually long gone." His message is the same as Blake. I suggest his "heavy feel" is Blake's pressure.
It may be that Blake is not right about keeping the ball on the face longer but his idea of keeping up the pressure through impact is certainly scientifically correct and the same idea as Kelley has in trying to have Zero Deceleration of the clubhead. Thus we get long shots for seemingly little effort by dragging the clubhead through impact with minimal deceleration. Keep up the pressure!
I think Mindy Blake's pressure idea had more proponents in the instructional field a generation ago than it has now. Today there are many advocates of "accelerating the clubhead through impact" in order to insure maximum speed at impact. Then there is the school of thought which says that applying pressure is not possible and that it makes no difference whether the head is accelerating, decelerating, or remaining constant. I favor the last viewpoint.
From my point of view, the question becomes: If Mindy wasn't applying pressure then what was he doing? Was he wasting effort? Was he mistaking another feeling for pressure? I don't have any good conjectures about this.
I think that Mindy was using the term pressure to describe what he felt as he dragged the club through impact. Many players feel the same thing and we all try to express the sensation in words, which are always a poor substitute for experience. Some players talk about holding the ball on the face through impact or other expressions and I think pressure is just Mindy's way of saying what he feels. The idea that the clubhead should not decelerate through impact is an old idea but I suspect most golfers or even scientists would agree with it even on putts!
Obviously, the clubhead is going to slow down once contact is made with the ball. No one on earth can maintain their clubhead speed so that it is the same prior to impact with the ball and after separation from the ball. Apparently, Mindy thinks the degree of slowing down during the contact phase can be minimized with his pressure swing. I do not think his assertion is accurate.
In any case, just prior to impact:
!) In theory what's the downside of decelleration?
The downside of decelleration is that the ball leaves the clubface at a lower speed and so doesn't go as far.
You are correct that there must be decelleration and the idea is to try to minimize it. Mindy is not the only one to try to do this and I guess my point is that this is what he means by pressure in the swing.
I've heard Johnny Miller say in interviews that his clubhead decelerates less than any other golfer's through impact. He credits his uncanny distance control and his pinpoint accuracy to this phenomenon. Although I don't believe his assertion is accurate it is nevertheless interesting and deserves some serious consideration because it comes from one of the masters of the game.
Miller thinks that our brains can do things with our bodies that defy scientific measurement. To him, that includes micromanaging impact to the thousandsth of a second or even finer.
I read Miller's book a long time ago and it was a very interesting read. He certainly believes one can will the ball into the hole. I have trouble just balancing it on a tee!
The feeling of accelerating the ball is as you say not real but it does result in hitting good shots.