One of the most important discoveries on a bike (as opposed to a car) is that shifting REALLY fast makes for smooth upshifts. The super-simplified reason is this: when you’re in a lower gear, the engine will be spinning faster than it needs to in the higher gear. In a car this sometimes means you need to wait a bit to get a smooth shift. On a motorcycle, the engine spins down so much faster that you need to be very quick to catch the engine before it revs down too much.
To make this more concrete, let’s suppose that you’re going from first to second, and the bike is currently going 12 mph. In first gear on a Ninja 250R, that puts you somewhere in the neighborhood of 5-6K rpms…let’s choose 5,500rpm to make it really clear. Now suppose that, to go 12 mph, you need to get the engine to 4,500rpm in 2nd gear (the engine will spin SLOWER in a higher gear at the SAME ground speed). That means that you have from 5,500rpm to 4,500rpm to clutch in, shift up, and clutch out. The problem is that the engine revs down that much in about half a second, so that’s all the time you have. If you go too fast, the bike will jump forward a bit when you get the clutch re-engaged (because the engine is going too fast). Usually, however, and especially when you’re learning, you go too slow because you’re not used to motions yet. When you let out the clutch (especially if you dump it), it’s as if you’re applying the brake…sometimes really forcefully.
I’m thinking of writing a more in-depth explanation of this…we’ll see if I get around to it.