Slip cap pens leaked. So when someone came up with a threaded screw on cap that also had an inner cap that seated to the section, it prevented leaks in your pocket and was called a “safety pen”. Some Swan pens with early screw caps are marked “safety pens”. Today, we tend to think of retractable nib pens as safety pens.
Wirt had another solution. There is a feed that is slightly mobile. A rod inside the screw cap will push the feed firmly into the section when the cap is tightened. This shuts off ink flow and prevents leaks. Unlike other screw cap pens, this arrangement won’t even allow leaks into the inside of the cap.
These pens were typically small, and made to be carried in a vest pocket. After all, a normal pen on your desk won’t need this feature. In order to accommodate the ink shut off rod, the feed is reduced to a slender gold underfeed.