The movie demonstrates that the volume of an oblique prism is equal to the area of its base multiplied by the altitude.
If we truncate a top of the prism by a plane perpendicular to a lateral edge (a perpendicular section), move the truncated part to the prism's bottom and glue both parts, then the oblique prism is transformed into the right prism.
The base of the right prism is the perpendicular section of the oblique prism. Lateral edges of both the oblique and the right prism are equal. As for the transformation, volumes for both prism are equal too.