A whipsaw or pitsaw was originally a type of saw used in a saw pit, and consisted of a narrow blade held rigid by a frame and called a frame saw or sash saw (see illustrations). This evolved into a 2-3 metre straight, stiff blade without a frame and a handle at each end. It was used close to the felling site to reduce large logs into beams and planks. Sawyers either dug a large pit or constructed a sturdy platform, enabling a two-man crew to saw, one positioned below the log called the pit-man, the other standing on top called the top-man. The saw blade teeth were angled and sharpened as a rip saw so as to only cut on the downward stroke. On the return stroke, the burden of lifting the weight of the saw was shared equally by the two sawyers, thereby reducing fatigue and backache. The pitman had to contend with sawdust in his mouth and eyes and the risk of being crushed by a falling log. 
Two men sawing lumber with a pit saw, the log on trestles rather than over a saw pit
A postcard of Chinese sawyers using frame saws around 1900. This method does not need a saw pit.
- The New Shorter Oxford English Dictionary (1993)