My mum today asked me if I "Double De-Clutched" when I shifted down gears (eg. 4-3, 3-2). I'd no idea what that meant, having used automatics most of my life. But after some searches, I found some videos which show it being (please do call me wrong if I am) increasing the engine RPM during the neutral change during a gear change, to match the equal value of a new gear when shifting into that gear. Is that right? I've never known it - if I shift down a gear, it's usually do it to slow me down before a junction or roundabout, so I actually want the additional engine braking that pulls me back. Otherwise, if I'm coming up on a red light or other "stop" point, i'm quite happy at 60 mph to put the clutch in, and just coast/wheel brake until I stop. In fact, I could sometimes just coast with the clutch in for half a mile or longer while traffic is gradually slowing down to a junction.
What is the actual use of this "double de-clutch"?