I developed the Wonderthumb to help with my own playing, as I never found a shoulder rest setup that did not cause me pain. And playing without a shoulder rest led to a feeling of insecurity in my left hand, which lead to tension there. Necessity is the mother of invention...
Shoulder rests can be a wonderful thing if used correctly. And the Wonderthumb is not necessarily a replacement for one. Many young students suffer from left hand issues even with a shoulder rest (pancake hand, thumb held in too tight, death grip..etc). I hope that the Wonderthumb may be of assistance for them as they get used to the dynamics of holding a violin properly.
My personal experience with using the Wonderthumb without a shoulder rest is that it helps me to find a natural balance for the instrument. With the body of the instrument placed on my collar bone, and the violin neck supported by my left hand via the Wonderthumb, I must stand straight and tall. I also naturally adjust the angle of the instrument left/right for best balance between my left hand and my bow. There is no need to clamp down on the chinrest, as the violin rests on the collar bone due to gravity, without the fulcrum of the shoulder rest levering it up. The neoprene Shiftpad on top allows for easy shifting.
My son began playing using a shoulder rest, and the added friction of the shoulder rest allowed him to hold the violin way down on his chest, his head leaning forward to hold it in, elbow down at his side. No matter how we cajoled him, that's the way he held it. It was enabling him to have bad posture. But the first time he tried playing without the shoulder rest, and with the Wonderthumb only, his posture straightened up significantly. My wife was shocked at the change. The violin had to adapt to him, not vice versa.
Almost every masterclass that I attended for my children while they were studying the Suzuki method pointed out having a good "mouse house" space in the left hand. That's the space that is filled by the Wonderthumb. Even with the ubiquitous use of shoulder rests, the cases of problems such as "pancake wrist" among beginners are epidemic. It would seem, therefore that a shoulder rest doesn't really automatically address many of the issues inherent in the violinist's left hand. I'm offering an alternative to the status quo. So that a student can get used to proper hand posture.
It seems to me that most new students are indoctrinated into using a different sort of contraption, called a shoulder rest as a compensation. There doesn't seem to be much instruction on what the purpose of the shoulder rest is, or how it is best utilized for the beginner. It seems to simply be installed on the first day of class, and that's that, they have to figure it out on their own.
And then do we wonder why we need classes on how to get our bodies back in alignment after years of playing? Yes, the shoulder rest can completely free the left hand from any support duties, but that support doesn't just go away, it gets transferred elsewhere--to the head and neck.
The hand and arm are fully functional and adapted to the task of holding things up. The head and neck are adapted to only holding your head up, and issues seem to arise when asked to do anything off axis, or one sided.
For me, that dynamic caused a great deal of actual pain. Which is why I came up with an alternative.
Some who have tried the Wonderthumb have said to me that it would be great as a transitional tool to learn how to go from playing with a shoulder rest to playing without one (for those who choose to go that route).