Ok, I'll play this silly little game...
There are many forces acting upon a vehicle in motion, but let's just concentrate on Acceleration and Drag (forward and not so much forward)
With the engine running, and in first gear, in idle on level ground it is wanting to go forward.
Everyone still with me here...good...there's more...
if there is a slight incline down (relative to the vehicle)
the engine is still trying to drive the vehicle forward,
BUT, the engine will only turn so fast at idle.
THUS (and this is the fiddly bit)
The engine cannot drive forward as fast as the vehicle wants to go...
The engine will only "DRIVE" the vehicle up to a certain speed.
Drag is induced, maintaining some semblance of control over the acceleration of the vehicle. (not considering other factors, this is a simplified illustration)
Think of it like this...
You take your dog for a walk, if you do not have one it's okay, this is a thought experiment, you can just make one up.
You put it on a leash because you are a conscientious pet owner, and do not want a repeat of the Poodle/Greyhound (the 5:45 for salt Lake city) incident from 1985.
You and your dog are walking at 5 miles per hour, there is no acceleration felt by you
(remember all things are relative...except your goofy aunt, no one claims her after last July and the potato salad incident, who knew a bikini could hold so much potato sala-oops, sorry)
then you stop.
the dog wants to go on.
Lets say you have a poodle.
(lets say you need a better dog...but you can upgrade later)
the resistance felt as the dog tries to go on are the combined forces of drag/acceleration.
you are stopped (or stopping) and the dog is attempting to go on...
Is the dog "Accelerating"?
Depends...remember it is relative.
Lets use the ground as a reference, it's there and it's free for our illustrative purposes.
If the dog manages to move relative to the ground, it is under acceleration.
with you holding it back it's under drag, the resistance factor that keeps it in check.
if you "throttle up" and run after the dog, you are "releasing the brake" and away the dog goes...straight for the Greyhound terminal and onto that damned 5:45 outbound for Salt Lake.
Ok its a basic illustration, but I didn't see a consistent frame of reference in the arguments presented.
And the argument that it wears out clutches faster,.I have no evidence that this is so. I've just pulled the engine out of my Ranger (1991 XLT 5 speed)
It has had exactly one clutch replacement in it's lifetime, and that was at 300,000 miles, I know because I did it. Did it NEED it? no, the throwout bearing was screaming, and needed replaced, so the whole assembly got uprated bits.
By the way, the engine let a headgasket go at 400,098 miles...so do NOT LECTURE ME about how it WEARS OUT AND ENGINE AND CLUTCH...it just does not if you use the clutch as intended, and do not slip it all the time.
I have however, seen them blow up, Guys going downgrade in a fullsize Chevy 4X4, Manual tranny in first, Transfer Case in low range 4X4, they hit 15mph and decide to give the engine a rest and push in the clutch...suddenly their braking force from the engine is gone...and the compound gearing spins the clutch beyond its ability to - well - spin. Makes a real mess of things, If you are lucky its only vehicle parts to replace, and not human body bits.
As far as shifting...
I've been a Professional driver for over 20 years. (and a lot of hands on vehicle drivers training on top of that)
So any discussion on any particular techniques I take with a grain of salt.
Having said that, Here is what I have done for pretty much all my driving life.
(I learned to drive stick in a 1957 Triumph TR3A so I started out with the good stuff)
When up shifting I pull the tranny out of gear without using the clutch, by easing up slightly on the throttle.
If I am familiar with the vehicle (as in only with my own equipment) I then feather the throttle down and put light pressure on the stick for the next highest gear, you do NOT want to push firmly on the stick until it is ready to go into gear. you just want to "feel the grease between the cogs" it will tell you when it is ready to shift.
you can run the gears all the way up that way, easier of course if you have a tachometer.
(shift points for your personal vehicle, learn them, it makes a difference!)
Downshifting is similar in technique,
ease up on the throttle (just like you are going to push the clutch in, only don't push the clutch in)
then slip it out of gear into neutral...
At this point you CAN go ahead and push the clutch in and complete the down shift, it is the SAME THING as double clutching only with one less motion.
you can feather the throttle up to match engine to road speed while putting LIGHT pressure on the gear shift lever. Remember, you are "feeling the grease..."
In anyone elses vehicle I shift the standard way, clutch in, shift, clutch out. (it isn't mine to do as I please even though I have proven on countless vehicles that its easier on the drive and engine)
The clutch should only be "slipped" when starting to move from a stop. Otherwise treat it like an On-Off switch. ESPECIALLY if you have a ceramic clutch!
those do not just wear out when abused, they BREAK! (it is a very expensive sound to hear a clutch go out)
Engine braking, either with a Jake or not, has saved my life on many occasions,
There are times where I have nearly gotten into trouble with it too!
Icy or slick conditions, you want ALL FOUR (or as many as you have) wheels providing a balanced slowing force, if its just the drive wheels (front or rear) and you are using engine braking, you run the risk of them breaking loose, In an 18 wheeler that means jackknife in no time flat because your drive wheels are right in the middle of what's going on and when they break free of their earthly bonds they want to get out in front and see what all the fuss is about. Rear wheel drive vehicles means a spin.
Remember, a skidding or sliding wheel will want to overtake, thats why drift cars can corner the way they do, the rear wheels slide out with more acceleration and tuck back in line with slightly less throttle. You can do it on a motorcycle too, its fun!
Since I'm flying my credentials here I'll add that I have also had many hours of Emergency Vehicle Advanced Driving Education (EVADE), I'm qualified to drive everything from fire trucks to ambulances and Enforcement vehicles (Includes armored trucks, dump trucks, straight trucks, bus' and 18 wheelers). at one time I could outdrive 95% of the general population, but age and caution has brought that number way down I am sure.
I have to sit back usually when these discussions come up, quite entertaining. I am NOT pointing fingers or casting blame here, but it is fun to watch some bring up "experiences" that they have had and banner it as gospel for all to obey. "Thou Shalt" doesn't usually work for me without consideration of the source.
I try and follow this basic philosophy when it comes to vehicles:
"Use the best parts you can afford, drive as efficiently as you can, help others as often as possible whether it be with wrenching, troubleshooting, or just plain listening."
And in my personal existence:
"Go Placidly amidst the noise and haste and remember what peace there may be in silence..."
to that sweet silence I now return...