Clutchless gear changes

[bales]

This is my first post so I will say hi to everyone first, I am currently in my final year of a mechanical engineering degree so I have a fair degree of knowledge on the workings of gearboxes and the like. However the one thing I have never managed to get my head around is how bike gearboxes and race car boxes can carry out clutchless upshifts without there being expensive graunching noises. If any one could explain this to me in "relatively" straighforward way I would greatly appreciate it, cheers.

Alex