The four pins

Two wires and two spares

The header is four right-angle pins, 2.54 mm apart, printed GND, NC, NC and SIGNAL. GND is on the square pad, and that pad is the only way to tell four identical holes apart. Count from it.

Pick a pin to see where it goes. GND is where everything on the board ends, the disc's black lead included. SIGNAL is the top of a 100 nF capacitor, fed from the disc through 1 kΩ. The two NC pins are in no net at all.

On most TinkerBlock blocks the pin after GND is VCC. On this one it is not. The disc turns a bend into a charge of its own, and the four parts on the board are a resistor, a diode, another resistor and a capacitor: none of them needs power. So there is nothing to feed, and two wires are the whole connection.

The pads that say VCC

Near the right-hand edge, the disc's two leads are soldered to two small pads. The black lead is on the pad printed GND. The red lead is on the pad printed VCC, and that label is the one thing on the board that misleads: the pad is the disc's own node, the point where the diode, the 1 MΩ and the 1 kΩ meet. It is not a supply and nothing outside the block goes to it.

The same two pads are repeated on the back, so the disc could be soldered on either face.

What SIGNAL does

SIGNAL sits at 0 V while nothing touches the disc. A tap lifts it within about a millisecond, and it drains back to 0 V over a tenth of a second or so. How high it goes says roughly how hard the tap was, so it is read with analogRead, not digitalRead. The first reading wires it up.