A plastic cover or fabric that covers the table, as the playspace.
The piezo transducers are sewn or sit within the middle layer of the fabric/plastic.(cover is made of a top layer, middle and bottom).
For each area of where the piezo’s are placed, there is a wider circle perimeter on the top layer. This would be a potentinator/resistor that when sliding your finger or thumb from the inside to outter perimeter or vica versa would affect the pitch change/tone of the percussive sound. broad example: https://www.youtube.com/watch?v=qA7mFhsfBMw