The technique works by starting with your feet together, pushing one skate out to the side while angling the blade so the inside edge carves a C-shape into the ice. As you push, you transfer your weight to the gliding foot and recover the pushing foot back under your body to repeat on the other side. Done correctly, you generate speed going backward without ever turning around.
Every defenseman needs this skill. Every crossover, pivot, and transition in backwards skating starts with a clean C-cut.