The torque driver is the rear pulley in the transmission, and is largely responsible for returning the gearing ratio back when the bike is decelerating (same as shifting down the gears in a manual transmission car or motorcycle). Without going into the full mechanics of how this is done, one half of the pulley has guided slots and the other half has matching pins which slide smoothly along the slots to allow the two halves to twist, opening and closing smoothly.
Over time, the pins wear grooves into the slots – which eventually causes the pins to get stuck rather than guiding smoothly the whole way along the slots, causing the two halves to get stuck. Essentially this cuts off the low end of your gearing, and causes the initial acceleration to be extremely boggy and slow until the bike reaches 20-30kph. depending on where the grooves are it can also cause the bike to initially accelerate well, and around 15-25kph suddenly jolt forward and drop dramatically in revs. This is extremely common on the Minarelli engine in particular. There is only one solution, which is to replace the outer pulley with the guide slots.
Fortunately Malossi make this upgraded outer pulley, made with strengthened materials to prevent this from happening again! On-top of that, this driver has 2 different sets of slots at different gradients 36° and 38° – other than for tuning, this also gives you a 2nd spare set of slots in case the first set wear out!