Fluxbox' tabs aren't in any way a new idea. The implementation is much like that of the PWM Window Manager. The way it works: Multiple windows are grouped together, and they share the same geometry- they're the same size, exactly in the same position, and moving one window results in moving them all. Think of them as a stack of papers. The tabs are like those little little plastic tabs you can stick on papers to make flipping to a certain page very easy and fast.
In versions up to 0.1.14 the tabs actually were attached to the window. From 0.9.x tabs are embedded into the window titlebar. Version 1.0rc1 and later has both options, configurable in the config menu.
That's exactly how tabs work in Fluxbox. Simply by selecting the proper tab for the window you want, and that window pops up to the top of that window stack. Now, let's try it.
The first thing to remember is that all tab manipulation actions use the middle mouse button. So, to start, pick two windows that you'd like to group together. Middle-click on the tab for the first one, and drag it onto the tab for the second. Congratulations, they're stuck together! You can use the tabs now to switch between the two windows.
To remove a tab, it's the same thing. Middle-Click on a grouped tab and drag it away.