[san rensho]

What you are doing is double clutching without using the clutch. As long as you hit exactly the right rpms, it will shift every time, but get it a little wrong and it grinds hard, and you are shaving off all kinds of bits on the gear teeth. Problem with your method is that it is very slow and very prone to error, and a big learning curve where you are damaging the gears.

I've done this when the clutch on one of my previous cars went completely. I had to start the car in 1st gear, bucking alll over the place, then shifting to second and 3 and back down while limping home. Not fun but doable.