Spy! Break! Inject! is a 4-wire Monster-in-the-Middle circuit. Inserting this board inline with any 4-wire bus lets you spy on, break into and inject signals. It's a purpose-built breadboard for reverse engineers.

The Würth terminal blocks have specifically been chosen to support wires down to 30 AWG for working with the smaller conductors commonly found in multicore communication cables. Right-angle pins and straight sockets allow connecting DuPont style jumper wires to other components such as microcontrollers or signal analysers. A row of test points along the bottom of the board support the spring-loaded hooks typically found on oscilloscope probes. All these connections are routed through a bank of jumper headers, allowing the paths to be broken and signals routed off-board.

Multi-meter probes can be connected across the rails to measure voltages. Current draw for a component can be measured by removing the jumper from the VCC rail and clipping an ammeter in line. Removing jumpers and connecting wires to the pin headers allows signals to be routed away to a microcontroller for logging, relaying, proxying and modification.