The idea is to accustom the bike,s engine to its stroke cycle in a smooth way.
All of you must have noted that during the running in phase, the bike,s engine sounds like raw, or "kaccha". like its not at its best and stepping on the throttle makes u feel that the machine is falling short of the challenge. so the idea is to go slow and let all the new parts, the rings and piston , get properly seated in the cylinder.
The other day, I saw an engine of Yamaha 100, collapsing on the first day as the inexperienced rider got a friend to ride with him on the newly tied bike.Simple reason....the engine wasn't ready to handle the brunt of the additional weight and gave in.
As you advance in the break in phase, you feel your engine gaining in power, specially after having done 500 kms and finally when you have done your 800 to 1000 kms, you know that your engine has developed to its full potential, as upon throttling up , the bike responds to the challenge rather than complaining and noising.
Again its always desirable to break in the engine at relatively cooler operating temperature and that's why old timers always prefer doing engines in winters.
The early oil change has two objectives to cater for:
to provide a safe lubrication environment to the new engine parts;
to allow the dirt and grime caused during engine assembly to pass out with the dirty oil.
I remember that I bought a nw cg 125 in 2000 and desperate to get over with the break in phase as soon as possible, took out the new bike for a long ride from pindi to lahore and back in 2 days. It was my lack of knowledge and certainly undesirable as I realize today and recollect how the bike had problems of achieving smooth acceleration to higher speeds for the rest of its service life.