@djogon has explained perfectly the transition mechanism that increases the volume during the release time of the input muting.
Before the arrival of GP4, I used to use a volume pedal to smooth out - when necessary - the attack of my guitar sound when changing rackspace.
Now, with the ability to use the input muting, the temptation is great to get a smooth morphing from one rackspace to another when necessary. I was used to using this crossfade function between presets on a Digitech multi-effects unit many years ago.
I recall that I have from a long time integrated this technique in my music within the same rackspace thanks to firstly Melda’s MRatio plugin then @Davidsan’s scriptlet x-fader and I can get now the same result between rackspaces thanks to the fade-in script.
I fully understand that this may not seem to you to be of essential use, but in the context of the music I play live, being the only instrumentalist accompanying a literary reader in total silence, the slightest difference in dynamics and sound level may be offensive. I try most of the time to stay in the same rack when it is necessary to do such a crossfade but sometimes I am forced to change rackspace when a lot of amps, fx and synths have to be replaced.
I don’t think it is necessary to continue this debate and to mobilize your time on this subject which remains very anecdotal compared to the expectations of the community. The essential point is that it works fine for me and I can even do without tweaking the rise time through a widget, just by typing it straight in the script. It concerns only a few rackspaces - though it is essential.
And I am really happy with GP, not only because of the functions it offers, but also thanks to the possibility to do things I would have thought impossible… such as this fade-in effect 
And many thanks again to all the devs and all those contributing to this wonderful adventure 