I wrote this in another topic, but i will also post it here:
Hey guys, i've done alot of thinking about how a contra spring works, and i think that i've figured out what the rpm are for when saying how strong the spring are.
Perhaps its because the spring will force the variator down in gear faster than the rpm will fall, so the rpm wont fall below 1500 on a 1500rpm spring before it is fully downgeared, and you have stopped. :p
This also matches with what you've been saying about the feeling of more power, as you would start at higher rpm after breaking without stopping. It also matches with what i've been told, that the spring doesn't have any purpose but to keep the belt tight, as the rpm would never fall to the lower limit, as this limit is below idle speed when using the 30kg/800rpm spring.
I'm sorry that i've been a little stupid to listen to, but i think i've fully understood how the spring works now, and it all makes sence.
Also that you've had better speed at hills, as the gearing will be just as low as needed, so you wont be pulling too big a load up hill.