Many isomorphic keyboards free up the fingers for more uses, notably jammers free up the thumb. The freed digit is then able to operate a musical device in 2 or 3 dimensions. Ken Rushton - written November 2010
There are two kinds of thumb controls we are looking at: a thumbpad made from a small touch pad, and a thumb-stick (jamstik) made from a joystick control. Both work.
For a brief overview, see this mini touch pad.
Here's what it looks like after the 1st assembly, but before refinement and styling.
It does not make an ugly hand any prettier, that's for sure.
Attached to my hand is the "JamStik", my tentative, if obvious and ugly name.
Above, Left to Right:
1) A brass rod lightly fastened to my finger with a loop of tape.
2) The joystick base from a Sony Playstation, bought from a thrift shop (you can also buy them)
3) A heat-reformed piece of plastic pipe
4) Double-sided Velcro
5) 8-lead wire, about a meter long, running back to the joystick. Software blends the joystick signals into the music production.
With this, I predict that a player could "build" a chord of arbitrary complexity with apparently virtuoso skill: the thumb can 'pluck" a heck of a lot faster than most people can press keys.
One could also do some interesting things with arpeggios, e.g. hold down an set of notes, then use the jamstik to play entire ascending or descending set, in a harp or koto style.