I don't know how they did it but I do know why. Sodium melts at 208 degrees. It takes a lot of energy to make the phase change from solid to liquid, so the valve absorbs a tremendous amount of heat at this point without a large change in the temperature of the valve.
I remember as an undergrad chemistry student being fascinated by the concept of something catching on fire upon contact with water. I got hold of a little (1/4" cube) piece of metallic sodium. I dropped in the sink and poured some water on it, and it did just what they said it would.
I was lucky I didn't burn the lab down or get booted out of college. It was a very dramatic reaction.
1977 C&C 30 Mk 1 hailing from Port Clinton, Ohio