I presume double-clutching is meant to match the engine rpm to the rpm of the transmission gears after you change gear, no matter up- or downshift.
When upshifting, with the accelerator decompressed and the clutch disengaged, the rpm of the transmission gears decrease as you shift into a higher gear with the mph hardly changing while the rpm of the engine/clutch/trans input shaft also decrease. So there usually is little or no noticeable jerk when you engage the clutch to connect the engine and transmission.
When downshifting, the situation is quite different because while the rpm of the engine/clutch/trans input shaft will still decrease as mentioned above, the the rpm of the transmission will increase as you shift into a lower gear with the mph hardly changing. This would make a heavy jerk when you engage the clutch to connect a low-rpm engine flywheel/clutch/input shaft with high-rpm transmission gears.
To prevent the downshift jerk, you cant just wait for the rpm of the transmission gears to decrease as the car slow down at neutral or with clutch disengaged, usually with the throttle closed. Or if you don't want to lose speed, as in racing, you can pause at neutral, release the clutch, blip the accelerator to increase the rpm of the engine flywheel/clutch/input shaft, then quickly de-clutch to shift from neutral to a lower gear and release the clutch to engage before the rpm of the engine flywheel/clutch/input shaft drop again.
Correct me if I'm wrong.
Quicksilver put it just right: The real problem is double clutching will never work right if you have to think about it. .....and practice until your brain is no longer involved.
