The two clutch shoe halves are held together by the coil spring and it uses centrifugal force to operate. Basically how it works is the clutch shoes get thrown out by the spinning engine. As the engine's rotational speed increases and the outward centrifugal force is strong enough (the spring is unable to hold the shoes together) the shoes grabbing the inside of the clutch bell which turns the pinion gear.
I believe the Redcat Rampages come stock with a 6k spring. The 6k is referring to the rpm (revolutions per minute), the rotational speed at which the engine needs to reach before it engages or makes contact with the clutch bell. The 8k is just a thicker spring which will make the clutch engage later. With the engine producing this higher rpm (8k) and more power, the launch will be more violent. Definitely the 8k/red spring is not only a solid upgrade but an inexpensive one for any off-road RC as it'll give you a big low-end boost of power, more "snap" off the line!