[Matt Monson]

Its about the speed differential between the free wheel spinning speed of the gears. In particular on the 1-2 shift if you don't go to 4000rpm the synchro will have to speed up the loose gear to make it spin the right speed to slip in easily. The less speed differential the synchro has to overcome to make the gear match the slider the faster and smoother you can shift. So many people shift too low in the rpms, which is why you see these posts where someone goes to the track and is amazed at how good it drives. Usually the solution around town is another 500-1000 rpm revs before shifting and it will be happier. The downshift gets all this attention regarding revs. The upshift needs attention too.