Well, this is my theory...
Inside the engine are, invisible to the naked eye, millions of microscopic hamsters in millions of microscopic running wheels all connected together. Now these are genetically engineered hamsters which feed on nitro fuel. So, when you press the throttle, they get to eat more, and run faster, so your truck goes faster!

But seriously... There are certainly a LOT of resources on the workings of 2-stroke RC engines. A good place to look with a lot of info would be www.rcnitro.com , or for a more general idea, look up 2-stroke engines on www.howstuffworks.com