As video description says:
"Modified polar planimeter of pritz concept.When the tracer point moves along a line (here the surface), the wheel follows a tractrix. Note that the path of the wheel is always tangent to the planimeter (here is the integration phenomen) The angle between the initial and final positions of the wheel is roughly proportional to the area of the surface. The constant of proportionality is the square of the length of the planimeter. The longer the planimeter, the more accurately the area is measured."