The ‘Contra’ spring (or torque spring) in your scooter provides the oppositional force to the variator.
A lower gearing ratio results in higher speed, and higher gearing ratio results in less speed. If the job if the variator is to decrease the gearing ratio for higher speed, conversely the job of the contra spring (and the rear pulley more broadly) is to increase it.
To put this more simply, installing a stiffer contra spring will have an almost identical affect to installing lighter rollers in the variator. It will allow the engine to rev higher through the gearing range.
Like all things to do with transmission tuning, this is a fine balance. The installation of a stiffer contra spring will often also require an adjustment to the rollers.
Malossi make a number of different contra springs, in different stiffness’s (measure in ‘K’ rating) and different physical sizes to suit different make and model scooters.
See chart below, for an in depth comparison of the different Malossi Contra spring options available for 50cc scooters.