There seems to be a number of articles on the internet and a general consensus on this forum that think grip tightening for forehands is about providing extra propulsion by holding the grip loosely in the fingers then tightening. In reality the effect of grip tightening can be likened to a boxer throwing a snap punch. Being relaxed then tightening on impact. The grip should not be held in just the fingers for forehand shots. It should should not move a lot in the hand when tightening. The palm should touch the handle. Heres a great slow motion video of the pros which clearly shows grip in the palms. https://www.youtube.com/watch?v=NUp5duS9pp8 Here's a video from Lee Jae Bok. While I don't like his videos on smashing I think most of his videos are good. He shows the grip in his fingers for quick grip changes but note he tightens for forehand which is correct. https://www.youtube.com/watch?v=5GJcdJm7UrE
Finger power is very subtle, not to be easily seen, even on slow mo videos. You don't have to consciously have a big gap at the palm, as long as you use a loose arm initially and tighten the grip into strike. For comparison purposes, the completely wrong way of *not* using any finger power at all is to hit with a tennis stroke. You know what I mean, we've all seen beginners do that.
I agree there can be misunderstandings. And there are slightly different variations of technique. The palm is a big area of contact with different pressure points. Holding the racquet handle fully within the palm is very restrictive. You'll see a lot of intermediate players like this hold the racquet very tightly as a result and use a lot of arm strength in their strokes. I agree that the handle should not be fully in the fingers either without touching the palm.