When you dis-engauge the clutch you are taking the engine load off the GB, if you are still getting rough changes then you need oil change or you have problems in box.
You blip your throttle to match engine rpm with the drivetrain so when you drop clutch after gear downshift you dont get "load shock" of the drivetrain/engine trying to all synchronize to the same speed (weakest point breaks first - Compression skids, popped input shafts etc etc)
Motorbikes use a slipper clutch to help take this load shock away...
Many years ago I use to have ignition module which could create a "misfire" so you could flat shift on upshifts.