A SEPIC converter can step a voltage up or down, but unlike the buck-boost, it does not invert the polarity. It achieves this by inserting a wall.
Between the input inductor and the output inductor sits a coupling capacitor. This capacitor completely blocks any direct current from flowing from the input to the output. If the switching stops, the circuit is physically severed.
But when the switch oscillates, the energy does not need a solid path. The violent chopping of the input creates an alternating wave of energy. The capacitor, while blocking the steady DC pressure, freely transmits this high-frequency AC wave across the gap. The second inductor then catches the wave and smooths it back into a steady DC flow.
The physical substance is blocked, but the energy crosses the wall as a wave.
