Of particular concern is the interaction between the cylinder bore and piston rings. Unlike most of the other lubricated surfaces in the engine (which are pressure-fed or splash-lubed), the bore relies on a very thin film of oil held in microscopic cross-hatches in the bore surface. When the engine is first assembled, these cross-hatches have sharp ridges, which means they will occasionally experience metal-to-metal contact with the piston and piston rings during the break-in process. You want this to happen: you’ve got to wear these peaks/ridges down so you can get a good piston ring seal and reduce friction, and you need to do it in a controlled fashion. high RPM’s and very high loads early on (when there are lots of these sharp ridges) will cause galling/melting, which will make it impossible to get good long-term results. This is why you need to stay away from high revs and foot-to-the-floor acceleration for the first several hundred miles.
A good break-in procedure will vary speeds and loads. A better break-in procedure will follow deliberate sequences to assure best results. A deserted backroad is ideal because you will want to vary your speed up and down more or less constantly. Accelerate at part load (not foot-to-the-floor) until revs hit the specified break-in limit (if none is specified in the manual, I’d suggest 55% of redline). Don’t baby it, give it some throttle: you need cylinder pressure to push those piston rings out agains the bore walls. When you get to the specified RPM, hold there for a few seconds, then release the throttle completely and allow the car to slow way down with engine braking (if you have auto transmission, select a lower gear so it won’t upshift and drop the revs when you let off the gas). This step produces strong vacuum in the cylinder, helping to draw more oil up past the lower rings and clear out any wear debris. When your speed has dropped significantly, repeat the cycle. Your speed will vary significantly, like between 30-50 MPH (give or take), so you don’t want to do this where you’ve got a lot of traffic.
Keep doing this until you hit the end of the specified break-in period. You’re not done yet though. If you want to be thorough, you need to break it in through the rest of the rev range. Now that the initial wear is done, you can safely hit higher RPM’s without galling the bores, but there’s still more wear to be done before you achieve a good piston ring seal and best fuel economy. Not just wearing the whole bore more, but also wearing the bitter ends of the bores, the part that the piston rings don’t quite touch until hitting very high RPM’s, when all the parts are stretched/compressed that extra thousandth of an inch. If you drive 10,000 miles while never exceeding 5000 RPM, then one day punch it up to redline, the 10K miles of lower RPM action will have left that tiny ridge of unworn cylinder at the end of the stroke - and now your high RPM excursion is pushing the piston rings over that tiny ridge. Bad idea. So once you’re past the spec’ed break-in period, start regularly running it up to high revs, putting your foot closer to the floor. Hit some on-ramps, make some passes. I would avoid absolute foot-to-the-floor acceleration and redline RPM’s until you’ve got at least a couple thousand miles on the clock, but work your way up to that as you approach that kind of mileage. You don’t have to do this every time the traffic light turns green, but definitely don’t shy away from it; it’s good for the engine’s longevity at this point.
Brakes have a similar issue. Pads and rotors with thousands of miles have worn to match each other, so you get good contact area between them when you hit the brakes. Brand new pads/rotors, despite being superficially flat, are not so matched. They need a few hundred miles of constant dragging and mild braking events to wear in to each other. Don’t be afraid to whale on the brakes if you’re about to have a crash, but if you can avoid routinely stomping hard on them for at least a couple hundred miles, you’ll be better off in the long run.