Tuna knobs are control knobs that stick to any touchscreen and let you control DJ and music making apps like you would with dedicated music hardware in the old days.
Tuna Knobs are basically a stylus in the shape of a knob, with a small axis in the middle and a suction cup on the bottom. Made to control (music) apps with virtual knobs on every touchscreen device.
So Tuna Knobs stick to your touchscreen using a micro suction cup and translate a turning movement to a touch command. Simply stick them on your screen and you are ready to start twiddling away. Tuna Knobs use conductive rubber to translate turning a knobs into the right signal for your touchscreen, not dissimilar to how a stylus works.
Tweetonig did the engineering of these nifty little knobs so that they can be mass produced.