Decoupling is a simple concept - quite simply disengaging the gearbox input from the motor and allowing a car to coast without the interia of engine internals. It's nothing new, our ancestors spent endless hours coasting their early motor vehicles down slopes in an effort to reduce fuel consumption, but it does have a marked effect on efficiency.
a new Boxster, and I bloody hated it.
I hadn't realised just how much store I set by being able to accurately judge a car's coasting characteristics with a gear engaged - normally the longest ratio on a motorway. You know the stuff, maintaining progress without using the brakes by anticipating and keeping a view as far ahead as possible. Well, now I do, because de-coupling kind of wrecks normal progress using this method.
On level ground, or a slight downhill section, you feel the powertrain release pretty much instantaneously as you back off the throttle. The trouble is, your brain expects a certain amount of residual deceleration at this point, and it never arrives - it's a bit like falling of a track onto wet grass, that sensation of almost accelerating again as you hit the green stuff. Of course you can adjust for this, but it takes time and distance, which isn't always available on a British motorway. Also, if the car(s) ahead of you are coasting, you have so much less friction that you inevitably catch them and have to brake. Irritating.
Re-engagement is a little sloppy too: there's a definite jolt in the car, and you have to judge the reapplication of throttle very carefully - too much and you'll encourage the gearbox to kick-down, which is most unseemly.
The upside was I thought I'd found a real gremlin in the otherwise impregnable Boxster package, but that proved to be unfounded. By hitting the Sport button, you kill the decoupling antics and the Stop-Start function - and you get faster shifts and a raised rev-limiter. On balance, I'd live with the latter to avoid the former, but the Boxster would be pretty much perfect if there was simply a button to kill the coasting alone.
By now you know the rest about the Boxster. It's the best car Porsche makes.