This is when a cartoon character moves without any visible elbows or knees, but his limbs aren't stiff, but rather bendy like rubber hoses. For examples, see pretty much any black-and-white cartoon. In cartoons made before 1930 or so, this was an intentional style, started by Felix the Cat animator Otto Messmer and Bill Nolan, which was meant to prevent the motion of the limbs in question from looking like they were drawn through a strobe light and flickering - the idea was that if you didn't draw joints, you could make absolutely sure that the limbs in one frame overlapped with where the frames were in the last frame. Higher frame rates, the development of Squash And Stretch, and an awareness of camera blur reduced the need for this as time went on, and cartoon characters all of a sudden