The reason for the "engine breaking" is due to the fact that the revolution of the engine not being match to the wheel/s rotation of your vehicle (a car in this instance). This is where the engine rev matching come into action.
Let's take an example...
Say that you're going in about 100 km/h (and the engine spins in 4th gear and decided to downshift to 3rd gear. When we downshift to 3rd gear, then obviously the engine or the speed of your vehicle will slow down say about 60 km/h (a guess).
So the slow down rate of 40 km/h is somewhat a breaking effect right? (you could achieve the same "effect" by breaking). What happens is that the "breaking" is now applied through your engine which is the reason for that not-so-smooth sudden twist that we feel.
But what if as we could somehow make our engine spin at a higher rate simultaneously as we move from the 4th gear to the 3rd. Now since the engine runs at higher rotations it can easily "catch-up" with the rotations of your wheels thus resulting less strain which ultimately resulting a smooth downshifting.
If you find the explanation being able to make you more confuse, I ask for forgiveness... I did my best. But by the time you watch below videos, I guarantee, you'll see what I meant :D.
Guys/gals, I got nothing to hide. I'm not an expert driver, and I'm a totally novice at this. Out of my enthusiasm I did a Google search sometime ago and came up with some nice YouTube videos that explains this method of smooth downshifting your cars... so I know a bit of what I'm talking about too (I have tested with my dad's car and yes it totally works... but it's gonna take some practice and patience to get there).
Few illustrations of engine rev matching (without using "heel-n-toe", don't worry, you can learn about that from the below videos).
I took these illustrations from a web site called drivingfast.net (if you have time, it's well worth the read). If you don't feel like reading :P, then just watch the videos... easy to understand, those videos ;-).
The expert drivers (especially those racing dudes) they actually are so good at this, they can downshift from 4th gear to 2nd, simply bypassing the 3rd gear!. But don't try it first, just watch these videos and take it slow.
Below is actually one of the best videos that I've come up with. It's from a guy called Greg who runs a driving school called M5 (you can also use this link to download all the M5 videos as in the below video window as well).
After finished watching the video go to this vimeo video page and in your web browser the other parts of this video serious which includes all the proper things that you should know before driving can be found to the right side of your screen. Seriously... Greg really knows what he's talking about.
The next video is actually a dude from Philippine, who's humble enough to say that he also learned his techniques from the above mentioned Greg from M5 school!. But the only difference in this video, you'll see a much better heel-n-toe video or superior video quality in general (oh we the young generation are pretty darn good tech geeks after-all :P) in comparison with the above video.
Not just down shifting, again while watching this video you'll also learn about up shifting and many other aspects of advanced driving techniques as well.
Below is the part two of the above video.
But please remember that, always be aware of the fact that you're not the only one on the road. So while "practicing" take extra precautions about other vehicles and importantly pedestrians (especially kids, etc). It would be better if you could have a more "open" road while practicing. Good luck.