The Lamy 2000 multi pen uses gravity to determine which refill to extend. I love this particular method because it always feels like magic when compared to the more common method of using one button per refill. How does the gravity system work? There are 4 refills inside the pen, and there are corresponding color indicators at the top of the pen barrel next to the nock (with the Lamy, the clip acts as the indicator for the black refill). Simply orient the pen so that the color indicator for the refill you want to use is on the top, press the nock down, and the corresponding refill extends. When you're ready to retract the refill, it works just like any other retractable pen — simply press the nock down again and it releases.
Pens that use this system can be a little finicky. For example, I have an old Staedtler multi pen that needs the orientation of the pen to be exact before extending the refill will work properly. Otherwise, it jams because two refills are trying to extend at the same time. In the case of the Lamy 2000, it works flawlessly. I noticed on the first few dozen operations that it felt a little scratchy, but that went away over time. I imagine there was some roughness or burs on the inside of the nose cone that were knocked down after some normal use. At this point, it's a smooth and satisfying experience.