It is generally a process of moving parts wearing against each other to produce the last small bit of size and shape adjustment that will settle them into a stable relationship for the rest of their working life.
In general, people no longer break in the engines of their own vehicles after purchasing a car or motorcycle, because the process is done in production (citation needed).
At idle speeds on a new engine poor machining tolerances could prevent the lifter from spinning and destroy the camshaft.
The break-in period required has changed over the years with improved piston ring materials and designs.
Those who promote raising the power settings steadily will recommend changing the engine setting from low to high powers as to not work the engine too hard and create excessive wear on the cylinder wall (which would require the pistons to be removed and wall fixed).
[1] A high power setting is relative to the vehicle type, so half as many rpm may be necessary if a car has a smaller cylinder wall.
For example, it is easier to hold tighter tolerances now, and the average surface finish of a new cylinder wall has improved.
Therefore, the average engine made today resembles, in some technical respects, the top-end custom work of back then.
Today engineers can confidently advise users not to put too much stock in old theories of long, elaborate break-in regimens.
[citation needed] Some users will not give credence to the engineers and will stick to their own ideas anyway; but their careful break-in beliefs are still harmless and serve roughly like a placebo in allowing them to assure themselves that they've maximized the equipment's working lifespan through their due diligence.
[citation needed] The useful side effect of a "break-in at slower speeds" for vehicles is operator familiarization.