somebody wrote something very smart:
You have two opposite functions to perform at break-in:
1) Apply enough pressure to the piston rings (via high-load) to have them expand and match the cylinder bore (in fact, they both wear a bit and fit each other). This also applies to valve to valve-seat interfaces and all plain bearings.
2) Avoid excessive heat that would cause distortion that will prevent these parts from mating.
BMW advise not to exceed 5,000rpm for the first 600 miles, for good reason, to prevent heat-seizures or cylinder scoring. This allows you to travel at around 75mph in top gear, throttle on - throttle off, which is fine for breaking in and doesn't require too much throttle restraint. My neighbour who is a mechanic for BMW, said if someone admitted they'd been red-lining it before 600 miles had been covered, they wouldn't be covered for any warranty claims for engine problems. Surely it's best to follow what BMW advise, after all it's only for 1,200 miles total before you can really wring its neck, if that's the way you wish to ride.
The main idea is to exercise the engine like you would a muscle. Build up slowly and don't do the same thing for too long. Yes, you want to keep engine revs down and increase gradually, but also you want to run a variety of engine and gearbox routines. Run City traffic for half an hour then country lanes for a couple of hours, then finish with maybe 15 minutes of mixed steady and fast motorway. Repeat until you've done the miles. Running motorways at 100 kph to do the distance will shorten engine life almost as much as not running in at all.
Here is an interesting statement from Jeremy Hall, a principal engineer in Honda's engine design department. He is quoted (page 61 of the July issue of Popular Science) as stating that the stress on an engine at 9,000 rpm is double that at 8,000 rpm. Something to think about as you wind your engine to redline.