The purpose of the middle joint pen is to move the joint away from the spot where you grip the pen with your fingers while you write. Sometimes fingers on an incompletely or poorly sealed section joint would induce capillary action and draw ink onto the fingers. The pen would not leak at rest, but it would just manage to leak on your fingers when you gripped the pen. If that does not make sense, picture how the pen does not leak when uncapped on your desk, but when you touch the nib to paper, the ink is drawn out on to the paper. The joint can act as an unintended narrow slit if it is not fully sealed. Here was a solution to inky fingers at the section joint.