If you are trying to "rev match" and your car is jerking it is because you are letting the clutch pedal out at the wrong rpm. Speed of engaging or disengaging isn't a factor, timing is the only thing to practice.

It is important to understand the concept of what you are trying to do when rev matching (heel-toeing is just rev matching while applying the brake). A person uses rev matching to make a perfectly smooth downshift which avoids any sort of rear wheel lock or untimely oversteer due to excessive deceleration.

When you are downshifting with rev matching you are trying to match the rpm of the engine to the rpm of the tranny in the gear that you are goin into. So the problem lies in knowing how high the engine has to be revving when you let the clutch pedal out at a particular speed. For example, using a downshift from 3rd to 2nd:
(plz excuse the km/h and the rpm and speed may not be accurate, it is just an example):
- You are cruising at 60 km/h and your engine is revving at 3200 rpm. You decide to downshift into 2nd WITHOUT rev matching, you feel your car sort of lurch as you let the clutch pedal out, and now you are in second going the same speed as in 3rd but your engine is now revving at 5000 rpm.

From this test you have just found out what rpm your engine needs to be at while rev matching from 3rd to 2nd going 60 km/h. So for this example you would need to "blip" (quick press of the throttle) and let out the clutch right when your tach drops down to 5000 (Letting the clutch out a hair earlier is better). Do this a number of times at a number of different speeds and soon you get a "feel" for it.
That's the best i can do to explain it technically, i hope it helps. Good luck and keep practicing, i've found that rev matching is the best thing i've learned about driving a car and there is nothing like downshifting while braking into a corner and executing a perfect heel-toe