Multi-instrumentalists: this is about personal experience (instead of literature review) of mentally practicing your instruments away from them. Do you find it to be more or less effective for one (type of) instrument over another?
[Real experiences only, please. I beg you not to answer if you are merely imagining yourself imagining.]
I'm a classically taught violinist and a self-taught drummer; "mentally practicing" drumming is much more effective than violin. I've made most (+50%) of my significant improvements in drumming first in just slapping my palms on my knees and tapping my foot. Of course, it might just be because you can actually practice drumming with your hands and feet. Violin is much more difficult to mentally practice since you don't have the same feedback about how well-placed your finger is, but it still helps. Also, one thing that I find very difficult to improve with from imagination alone is keeping tempo.
Regardless of the instrument, I tend to have the same habits and mental processes when imagining myself playing an instrument as when actually playing it. For instance, when playing difficult parts on the violin, I will subconsciously and immediately flare my nostrils - probably conditioned into me over the years. Imagining myself playing those same parts also makes me flare my nostrils.
In my freshman year of high school I was the backup drummer in our school's jazz/pep/orchestra bands, and quite frankly, I sucked. I couldn't use my limbs independently and I couldn't read music, but I was so into it. I memorized everything the lead drummer was doing, memorized the songs we played, mentally captured everything.
I was too poor to own my own drums, so over the summer I couldn't practice, but it was in my head, and I went over it over and over and over and over again.
First day of band practice my sophomore year, I discovered that the lead drummer had left and it was up to me. I sat at the drumset, and the entire band looked at me questioningly because they knew I wasn't up to the task.
And I rocked.
Probably the best moment of my life, actually.
This is a very difficult question to answer from a real experience point of view. For example if you measure your improvement by mentally practising how can you tell whether you would have had the same improvement by actually practising. I know you said you didn't want literature reviews and for that reason I wont go into naming sources, but there is sound evidence that mental practice creates the same neural pathways as actually doing the exercise.
For guitar, rhythm strumming can be practiced almost anywhere and it effectively translates to your strumming hand. Picking is a different story, but never tried it. I practice rhythms in my head and with my strumming hand all the time.
Fretting is not really something you can do without a guitar or some fretting device. It involves hand movement within specific ranges and is tied to muscle memory. That is why I am going to give the Halo Sport a try to see if it smooths my fretting by enhanced muscle memory (different post). Will update that by end of week.
Fretboard memorization/recollection, chord construction, scale recollection, etc are all things that I often sketch in my notebook at work while in meetings that drag on. And that is half of fretting and playing using the right brain. This does pay off when I am working out a progression that I heard, but applying it is still a matter of muscle memory... I just recollect it easier.
