Power from the panel hits Common on one switch. The other wires (travelers) carry that power to the other switch. That Common goes to the light.

Those particular switches have the Common at the bottom, one lone terminal by itself. Take two switches just like them, hold them in your hands with the double terminal ends touching, and the Commons on opposite ends. Stand between the panel and the light. Visualize power from the panel coming to hit a Common, the travelers between the two switches, and the other Common going to the light.

Easy way to teach a guy how to replace them: pull both out and separate the six wires (three per switch). Cut the power on and find the one out of those six that is now live. Put that one on Common and put the other two in that box on the other terminals. At the other switch box, one of the three will now be live; it’s a traveler. Go back to the first switch, flip it, and now a different wire will have power; that’s the other traveler. The third wire never has power no matter what you do to the other switch, even if you just tied all three wires together. That’s because that wire doesn’t go back to the other box; it goes to the light. It’s the Common for that switch.

So, when you’re installing them…you’re looking for what to put on the two Commons: one guy is working all the time, and the other guy never works.