It's easy to understand how an electric rc car works. You just put a pinion gear on the motor and a spur gear on the transmission and add current to the motor and it goes. Well it's not that easy to understand with a gas engine. Actually, I couldn't wait to get my truck just to see how it all works. So now for your viewing pleasure, you don't need to wait on your own car to see what makes up the clutch of a gas engine and how power is applied from the engine to the transmission.

First off you can get an idea on how the pinion and spur gear comes together as in any other rc car really. (Yea, these are worn out tires!!)

Also with this photo, you can see part of how the linkage hooks up for the break and the throttle

This shows you what the inside of the clutch bell looks like and to the right you can see the clutch assembly that is mounted on the purple flywheel. The spring is the secret behind this 4-n-1 clutch sold by MIP. What you can't see is special weights on the end of the clutch. This is what  helps hold the clutch out on clutch bell as the rpm's start to come up. The spring as you guessed keeps the clutch from opening while the engine is at idle. Some RC clutches don't have a spring. This just makes sense to me though to have one.

This last picture shows all the componets which makes the clutch system work.
To the left you see the clutch bell which is actually the pinion gear. In the middle of the photo you see the clutch off of the flywheel. This is a nice shot of the clutch from the rear. The holes are how the clutch connects to the flywheel. As you can see on the flywheel there are two pins though the lower pin in this photo is obstructed by the bottom of the truck. The clutch pushes right on the pins of the flywheel. As the rpm starts to come up on the engine, the clutch shoes start to move from the center out and grabs onto the inside of the clutch bell (pinion) and that's about it. From this photo you can see the weights that are at the end of the clutch. You are looking at two clutch shoes which is held together by the spring that you seen from the earlier photos.

Hope this helped