When shifting a trans without syncrhonizers, as in some older trucks, you push in the clutch, rev the engine, then clutch again and shift. Its a way to try and match engine to trans revs to ease the shift.
However the following is probably more appropriate to your question.
"Double-clutching is the proper way to downshift at speed without placing excessive wear on the transmission's synchronizers. This allows you to select a much lower gear without the tell-tale lurch you normally get when the clutch is let out after downshifting. "
"When downshifting my BMW M5 from 4th gear to 2nd at 50 mph, I need to raise the engine speed from 2200 rpm to 5000. To prevent excessive synchroniser wear, the clutch is depressed and the shifter is moved to neutral. The clutch is then released, and the gas is depressed to bring the engine speed up to where it needs to be for the lower gear. The clutch is then depressed again and the shift lever moved into the lower gear. When the clutch is released the second time, the engine is already turning the proper speed . While this sounds complicated, it's easier done than said, and only takes about a second. "
http://www.driversedge.com/dblcltch.htm