Anyone who's done much research on Feral druiding, especially cat form, has probably run across the concept of powershifting. Basically, powershifting plays off Furor (a talent that pretty much every Feral druid has) by shifting out of cat form and straight back into it to reset your energy to 40. If you do this at 80 energy, of course, you've just lost 40 energy, some mana, and a global cooldown, and there's no point to that. But if you do it at 0 energy, you've just traded a GCD and some mana for 40 energy, which is often worth it. Powershifting was made even easier in a recent patch with the door opened to macros like "/cast !Cat Form", which shifts you in and out with one button press instantaneously.
This no longer works in the current LK beta build. Now, as explicated at length by blue poster Jimmythenumbers (who we here at WI have never heard of), Furor works by setting your energy to the amount you would have if your energy regen had been working while shifted out, up to a maximum of 40 energy [reworded for clarity]. For instance: