Explain the dreaded rough down shift please
 

Way to much time out of the saddle leads to meandering thoughts. So I was wondering. When the tranny shaft splines wear/fret it becomes more difficult to downshift. Especially 3rd to 2nd. But the upshift isn't notchy. Why is that?

Difference in speed between the incoming and outgoing shafts of the gearbox.

Dry splines will definitely make the downshifts harsher.

so thats why rev matching on the downshift helps.

Yes sir. :)

Try holding the clutch until the engine idles, then attempt an upshift. It'll bang just as hard. If you shift up normally, rpm's drop as you reduce throttle and with the next gear requiring ~1000rpm less, you are automatically matching the speed of the ingoing and outgoing shafts.

When you down shift, your rpm will actually be ~1000rpm higher than you was, so reducing your throttle when you clutch actually makes the difference bigger. Blipping the throttle at the moment you switch gears to a point higher than you were before the shift will match your speed again.

With practise, you can even do a clutchless downshift by cutting the load briefly and quickly and tapping down, but my experience is that it only works very smoothly when you want to accelerate.