From the AcoustaGrip website:
The latest unique design in violin and viola shoulder rests. The AcoustaGrip Shoulder Rest attaches easily underneath your instrument and adjusts to the shape of your shoulder. A non-clamping, non-stiff, non-muting, ergonomic foam shoulder rest that will not fall off, that can be removed, and sticks to the instrument without glues.
AcoustaGrip products are made from Acoustical Foam layered with a patented layer of Air Pockets and contain no clamps or glues. These Hyper Nano Air pockets compress when placed against the back of the violin, causing the lightweight shoulder rests to attach gently and securely with Air. They can be attached and re-attached repeatedly. Another essential feature welcomed by parents and teachers is the fact that they can be attached and left on when the instrument is stored in the case. They compress for storage and then have a memory to reform for use when the case is opened.
The AcoustaGrip increases volume and overtones by not clamping onto the edges. When you Clamp wood it stops the Natural Sounds and Overtones. Old style rigid, brace-like “Clamp-On” shoulder rests suppress the sound, may injure your instrument and your collarbone over time. Clamp-On shoulder rests will also restrict your freedom of motion. Most Great violinist never used them and neither should anyone.
The AcoustaGrip Shoulder Rest is different - it uses a unique sticking surface. Proper shoulder rests should be able to adjust to the contours of your shoulder. AcoustaGrip’s Acoustical materials form to the shape of your shoulder, allowing greater range of motion and wonderful comfort. They give great support for holding your violin with ultimate comfort.
This is how AcoustaGrip works: Violins and violas are merely sound amplifier boxes. The things that control the sound output are: The sound post, base bar, and importantly the side and edge blocking inside of your instrument. These magnify and direct the sounds made as the bow strokes the strings. When you clamp any part of these components of your violin or viola, especially the Edges or internal Blocking, you will dampen your instrument’s sounds and projection. The AcoustaGrip Shoulder Rest uses no clamps, glues or rubber bands and attach securely to the bottom or base of the instrument, and do not interfere with all of the instrument’s major internal or exterior acoustical parts. That is exactly how the great Master Violin makers designed violins hundreds of years ago.