A motor spins one way. How do you make it spin the OTHER way with a fixed battery? Four switches in an H: close the top-left and bottom-right and current runs one way; close the other diagonal and it reverses. But close both switches on one side and you’ve shorted the battery dead. Slide through the states and find the two that work.
Two legs, each a top and bottom switch, with the motor slung between their midpoints. FORWARD = top-left + bottom-right (current down through the motor); REVERSE = top-right + bottom-left (current up); opening a diagonal lets the motor coast or brake. The lethal state is SHOOT-THROUGH: both switches of one leg closed short the supply straight to ground. A fail-loud self-check enumerates all 16 switch combinations and throws unless exactly two are drive states and every same-leg pair is flagged shoot-through — the rule every motor driver enforces with dead-time.
Idealised switches (no MOSFET body diodes, no dead-time timing, no PWM) — a real driver adds all three so the two forbidden states can never briefly overlap during a transition. The state classification and the shoot-through danger are exact.