Holy crap! How long is your uncle’s arm???
I was at that safety class for an article we did for Car & Driver. The method works fine so long as you don’t panic and do anything stupid. Other advice given was to turn on your headlights (so the rescue teams could find you should anything go wrong) and that whole “air pocket in the front of the car” thing doesn’t work, as most cars have the weight in the front. Hence, when sinking, you pitch forward and the air pocket moves to the rear.
I don’t know how long it is (pretty long since he’s 6’2") but it’s very big. Both of my uncles were weight lifters and could’ve played pro football except for injuries.
Actually, yes it will. The sideways pressure of the water can strain the ability of the window to roll up or down.
Consider a throw rug in your home. You could grab one edge and drag it anywhere with ease. But, if your dog is laying on it, it will take a lot more effort to drag it around. And if your sofa is on the rug, it won’t move at all.
Or, you could try a quick experiment with your own car. Roll up the window and have a friend lean on it. Now, try to roll down the window. But, don’t try it for too long or you’ll burn out your electric motors or strip the gears in a manual crank.
Ditto. I’ve been on the receiving end of a fist through a car window. Long story, but the short version is that I was sitting in the passenger seat of a friend’s car when a rather angry stranger punched through the window and connected with my head. Didn’t do any physical damage though. (Just to be clear: the driver hadn’t rammed anyone, intentionally or not.)
The window glass (not laminated) showering over me didn’t hurt. My skin wasn’t broken, although it wasn’t pleasant washing little bits of glass out of my ear and hair. Then again, if you’re at the bottom of a river, I’m sure there’ll be amble opportunity to have a good scrub on the way up. 
Incidentally, I’m mildly horrified that driving into a body of water is apparently so common that there’s classes and procedures set out for escape.
I always thought that, rather than a hammer, you should carry a center-punch. This is a spring loaded tool that, with the press of a button, shoots a pointed metal object out with great force. This object, however, is not a projectile. Instead, imagine a very assertive ballpoint pen. This suddon impact on the glass should introduce a fault that will shatter tempered glass. I do not actually have such a device, but I probably should since both of my cars have electric windows. One of my cars is a minivan and the windows on the sliding doors don’t open at all, and the rearmost windows only tip out. Still, wouldn’t a sliding door be easier to open under water since you only have to fight the pressure for a few inches, and you have the sliding track helping you along?
Thanks Flash, I was about to say something to that extent, but finals are starting in a day or two and I’ve been studying :(.
Anyway, I remember seeing a few programs on cars going underwater and the sort and most of the recommended using spring-loaded center-punches since those would be easy to use underwater. As for opening a sliding door underwater, it does seem like it should be easier. I suppose the only way to be sure would be to test it yourself ;).
I have seen this scenario experiemtned on tv.
This was all done in controlled conditions, the vehicle had no forward motion, and it was dropped into the water flat.
The window simply could not be opened when the car was on the bottom, but it did slowly fill anyway.
They tried escaping using the classic, let the vehicle fill up with water first to equalise the pressure and then open the door to swim out.
The window was left open and the vehicle sank, however the the door could not be opened on the descent to the bottom, because the pressure was increasing all the time.
By the time the vehicle had stoped sinking and was on the bottom, the car had been filled up for long enough for the driver to have been drowned had there not been an emergency breathing set.
The door could only be opened when the vehicle was on the bottom, and filled with water, but getting to this stage took way too long, the driver would definately have drowned.
They then tried getting the door open as soon as the vehicle hit the water and before it had started to fill up, this worked but it was a struggle, you might have expected the car to partly capsize due to just one door being open, it didnt.
Their conclusion was that if your vehicle lands in water, open the doors asap and get out soonest.