I did this mod recently in my KBD8x MKII and was pleasantly surprised by it. It’s a nice mod, in that like the tape mod, it can be done with materials you likely already have around the house and it is easily reservable. In my case, I used electrical tap on one side. If using a thinner tape, like masking tape, I would probably recommend two layers on each half.
I was interested in trying this mod mainly due to the same reason that @pixelpusher mentioned. The KBD8x MKII suffers from the same pinging on the top case that is present in the Brutal line. I have done some experimenting with the KBD8x, and really liked the o-ring style mount for similar reasons. The o-ring cut down on the ringing sound, probably due to the giant rubber band separating the two halves. It offered better sound in the case, the only issue is that the 8x has hardly any room in the bottom. Sure, you get better sound but on a half-plate (with the flex cuts in the PCB), your PCB will hit bottom.
Prior to this mod, my preferred set up in this board was half-plate (top mounted) with a thin layer of PE foam in the bottom of the case to remove hollowness. After the force break mod, I am happy to say that I achieved the top case NOT ringing when I tap on it, and it removed enough of the hallowness in the case that I did not feel the need for the thin foam on the bottom. This aspect is what I was surprised by the most. I don’t think it removed all the hallowness, but it was like 80%. I got rid of most of hallowness, but I also got a little bit of sound “character” back from the board (and a little more flex without hitting the bottom).
I think like the tape mod, the force break can be a big boon, however if you do it to all the boards you can, you are going to end up with a lot of same-y keebs.
As for why I think this mod is useful, I think in the KBD8x, the top is just thin and cheaper aluminum. Or maybe not cheaper but definitely thinner and without the brass insert present in the bottom to soak up any ringing resonance. My thought is that the top was were they cut costs, the downside is that it ends up acting like a ringing bell. I think the Brutal is much the same way, top is thin compared to the wedged aluminum present in the bottom. I am of the opinion that the force break mod just assists with stopping the sound from passing between the two halves. Other keyboards might not face these same issues if: their two halves are thicker, or they are maybe made with a higher grade, dense aluminum?
Honestly if you have a board that you can use this mod in, I would recommend giving it a try. I