He said it well above.
Basically a single cylinder about the size of a 350 chevy motor creates a lot of resistance for the starter to get it up to a nice fast starting speed.
The decompressor allows some blow by so that the motor can get up to speed for easier starting.
Many dirtbikes have single cylinders and use different decompression setups. Some have linkage to the kick starter that causes it to open when kicking.
Others have a lever on the bars such as the bigger Honda XR series. That is what I plan on using on my build. The foot actuated thing is a bit of an eyesore and clunky.
Here is a pic of one...
http://i15.photobucket.com/albums/a389/marklyons/2685171e.jpghttp://www.dirtbikebitz.com/images/products/controls/levers/RFX_CRF_Clutch_Le...