Cars have synchromesh . . it's like a cone shaped clutch between the dogs which get the two gears up to the same speed before engaging. Bikes don't have synchromesh largely because they don't need it. Not sure why, I'd hazard a guess at it being because they're sequential, the inertias are very low (except Harleys and old Guzzis) and the ratios are so close.
When going up the box, there's just enough of a slowdown of the input gear to allow time for the unloaded dog to slip into the window of the gear it's about to slide into, but when downshifting, we blip to unload the transmission and match the input and output gears to get that window. Which is why downshifting without a blip often gets a clonk too.
I think . .
Having Google it, I see there's not a lot of pages which explain this precisely, not that I'm 100% sure I've explained it correctly anyway!
