Initiating unit (ENIAC)
The panel with the start button: it cleared the machine, brought the supplies up in the right order and fired the first pulse out into the cabling.
ENIAC was forty panels, and this was the one you touched last. After days of replugging, the initiating unit did three things: it controlled the power supplies and the sequence in which they had to come up, it cleared the accumulators and counters before a run, and it sent out the initiating pulse that set the first program step going. Without it you had a room full of tubes glowing at nothing in particular.
It also handled stopping, which is where it earned its keep. Together with the cycling unit it allowed the machine to be run one addition cycle at a time: step forward once, walk the row of accumulators reading the neon lamps on their front panels, and compare that against what you had expected to be there. Debugging by looking at the hardware was not a fallback but the only method available, since there was nothing else to look at.