How does a pit saw work?

The pit-saw was a system used in colonial America and Canada, and in many other countries, to make planks from tree trunks. A pit about 6 feet deep was dug, and the logs to be cut were trestled over it, Two men would use a huge saw about 8-10 feet long and over a foot wide, large teeth were in groups of 3-4 and were angled to cut fast and remove waste each sweep. One man operated the saw in the pit, with one on top. It worked well and much use was made of these saw-pits.

I've actually seen this in operation at Logging shows around the Pacific Northwest and two strong loggers can make short work of trunks with this system !