How did a plastic pen save astronauts from being stranded on Moon in 1969?

On July 20, 1969, after first Moonwalk, Neil Armstrong and Buzz Aldrin were climbing back into lunar module when Aldrin hit circuit breaker switch with his backpack, breaking its tip. Aldrin then used a black felt-tip plastic pen to complete the circuit, allowing engine to fire and astronauts to safely leave the Moon. The pen was recently auctioned for $857,600.