hatchet planimeter !enter image description here How does this instrument work? Here is a video that demonstrates its use. After reading the wikipedia page, I still have no idea how it works. Any explanations that are easier to comprehend?

See Robert L. Foote: Geometry of the Prytz Planimeter. See also Mark Levi and Serge Tabachnikov On bicycle tire tracks geometry, hatchet planimeter, Menzin’s conjecture and oscillation of unicycle tracks, and Tom Apostol and M. Mnatsakanian: The method of sweeping tangents.