It's pretty much as Flukester says. If the gearbox is under no additional load, i.e. you're not accelerating or decelerating, a small amount of upward pressure (barely any) will allow the next gear to engage by quickly rolling off the throttle. There is no strain on the gearbox whatsoever and it provides an extremely smooth and quick shift. Clutchless down-shifts can be done but I would recommend you don't bother. It can be painful.