I've never taken this approach, probably because I had instructors, and I had to play the sheet music, as written. Part of this training involved utilizing dynamics, from
ppp to
fff and everything between. If this dynamic is written on the sheet music, then that was the dynamic that I had to play.
This approach did not please my poor mother, back before we had electronic drums. I remember her asking my instructor, a man who would later be my college professor, if I could "just practice it quietly and then play it regular at the lesson."
I remember him telling her that this would be a waste of time and money, and that if she wanted me to do this, then she should consider just mailing him checks and not bother bringing me out anymore. "King Richard" Paul was sometimes a "Whiplash" kind of guy.
This justified my request for a practice pad.
The concern associated with imaging the surface in any place other than the surface is that this can lead to bad habits. Imagining it deeper than it is to get louder, or higher than it is to get softer, seems like extra steps.
I saw a drummer last weekend in a small venue with cement floors and cement walls. He was hitting so hard that it was unbearable. He was a good drummer, but too damned loud for the building. Conversely, a few weeks earlier I saw
David Raven drumming with JackiO, and he was barely swinging the sticks, and yet came through clearly, even though the amps were cranked up.
EDIT:
Here is where you can see David more clearly.
Having the control to mindfully generate the proper volume and dynamic for the song, and then be able to adjust it depending on the venue, definitely takes work.
If the "timid factor" is related to performing in front of others, then that's a completely different beast to overcome. My very first performance in front of a large crowd was in 1980 with my high school band. The drum "line" was tiny, and I was the ONLY snare drummer.
There was no place to hide on that big track in front of almost 14,000 people. Plus, being outdoors and alone on the snare presented the challenge of being loud, and yet still maintaining accents / dynamics.
And you kinda have to dance a bit, too.
So I guess I'm skeptical of the "imaginary surface" approach. As my instructors have always said, from back then up to yesterday,
"Perfect practice makes perfect."