Standard chainsaws have a tension bar that rides inside the loop of chain, and the teeth cut to the outside. There are ‘survival chainsaws’ that are a variation in which the saw chain is passed around the thing you want to cut and the teeth cut to the inside of the curve as you pull the saw back and forth.
I seem to remember a third variation but I’m not sure if it’s real or my imagination. In this version, a big loop of ‘regular’ chain (not the pinned links of a chainsaw chain) is passed around an object to be cut, and secured to a sprocket on a big motor. The motor starts up, the chain is dragged around the object in only one direction, and the chain slowly grinds its way through. The chain is tensioned somehow; either my pulling the motor back or by letting it hang by gravity.
I seem to remember seeing video of such a system used to cut beached ships in major shipbreaking operations, and I also remember seeing a video of such a thing being used to demolish a building. But my Internet search skills fail me now, and I’m beginning to think that the whole thing was just something from a work of fiction. Am I remembering this correctly? What is such a thing called?