Most people think that the engine bores are smooth and shiny, in fact some if not all car manufacturers go to very great lengths to make sure they are not, and all recommend that you work the engine within a certain rev range with plenty of gear changes etc.
If you run a new engine too fast it will develop hot spots where things are slightly tight and this will accelarate the wear process dramatically.
Conversely if you run it too smooth and steady, the bores themselves will burnish and polish, becoming extremely smooth, which is also not desirable.This is also known as ‘glazing’.
It is very expensive to rectify requiring an engine stripdown, a mild sandblast in the bores and new piston rings.
The smooth burnished surface is not able to hold oil properly and so wear increases, oil consumption increases and emissions become dirty, extreme cases could total the catalytic converter.
Other contributary factor is oil, modern oils such as the close tolerance low wear synthetic types are so good that they can extend the service interavals without any further redesign of the engine, but they also reduce wear hugely and during the running-in period this prevents parts breaking in properly.
It is a good idea to use plain and cheap engine oil for the running-in period and then change to a better grade once this has been achieved.
I have read that one way to speed this up is to drive to the top of a good long hill and descend in a lower gear than you normally would, and let the engine do the braking. This apparently creates a vacuum which splays out the piston rings.
I would also mention that this was recommended by a race bike guru and it may be either nonsense or only applicable to bike engines, maybe someone could comment.