man i personally dont like the term feather when it comes to clutch work. feather suggested a constantly varying amount of X. Feathering the gas is what you do to negotiate a turn at the limit. feathering the brake is what you do to maintain threshold breaking. once you start letting the clutch out you shouldn't be increasing anymore, only continually decreasing. Contrary to the previous post 'slip' has been the term i have always used to describe a clutch doing what its supposed to be doing.
as a note to Vew's post. If you are doing correct double clutch down shifts you should be able to engage and disengage the clutch instantly (or as close to 'instant' as physically posible). in other words, you should be able to drop the clutch on a down shift, regardless of gear. Also "when you down shift from 4th to 3rd, the engine RPMs won't be much different" is extremely misleading. A car doing 30mph will have a much higher rpm in 3rd then it would in 4th. The fact that the RPM's should stay relatively close during the down shifting process is a function of car decelerating. if i was simply doing a double clutch down shift with no intention of slowing down my RPM's will increase. They should in fact stay exactly where i blip them to while at the neutral stage of my double clutch down shift.