Opposite to ecthe, I find that most peoples problems in getting good spin on serves is that they don't hold the racket tight enough. Hold it as tight as you can and try a backspin serve. You'll find that the ball enters deeper into the rubber.
Then loosen up to the point where you can flow more freely.
Then loosen up and tighten up only at the point of contact.
I have never found that concept to work on serves. However, on strong impact / powerful shots, I find that firming up the grip right at impact allows you to deliver maximum force that will amplify your rebound. That concept you discus is IMO big-time essential to a powerful shot.
With the serve, we are catching, accelerating, and throwing the ball almost perpendicular to the bat. The firming of grip works for when we are trying to make the most rebound, which translates mostly in pace and some more spin.
You get more acceleration and terminal speed of bat by being loose.
There are a number of ways to visualize things.
I frequently tell people to set themselves up all the way around the BH corner right up to the table and look down the endline. It help keeps one in position. One uses the endline as a baseline guidline in their ball toss and impact. Using the endline makes one subconsciously much more consistent in where they impact the ball (and toss it)
Sometimes it helps to imagine you are impacting the ball under it and a bit to the FRONT of the ball. That helps one keep the bat angle open so they strike the bottom of the ball. Too many players close their blade on the backswing and never open it enough to get under the ball, then do all kind of crazy stuff to keep ball short - usually it is swinging down or stopping their stroke to slow down the rebound that is already too direct.
Serving pendulum with tip down and bat full open is a great way to initially improve impact consistency.
Often, a player tries to practice with a real short toss and a real long backswing and swing. I am the odd person that recommends a higher toss and a shorter backswing and swing. The part of the whip where one generates all the bat speed needed doesn't take a lot of space, maybe a foot or so is all that is needed. maybe later, when the main part of the swing timing is good, a player could profit from learning a variation with a long swing. I believe at first, it is too much to work on and get the timing of that long swing to impact the ball at the right time and way. Same for the short toss, not enough time for a player to setup and go into action. I see player after player initiate their swing too late and with a short toss, it is already over.
Once one gets the basics to a real high consistency, they are ready for a few small things that improve their impact and recovery. It is better to work on those later, after one is real good on impact dynamics.