The Turnstile pen has a fairly standard button filler arrangement under the end twist cap. The button, however, is flat, wide, and sloped on both sides to facilitate two opposing ramps or cams under the end cap. Twisting the end cap causes the button to be pressed until the ends of the ramps are reached and the button can snap back. Internally, this utilizes a standard pressure bar.
The Conway Stewart Speedy-Phil uses the same general concept, but hidden under the barrel and inside a metal tube.