This is not the first time that a
 Lego CVT  appears on this blog, but this one uses a different mechanism than all the previous appearances of this kind of 
gearbox, it uses a 
chain, like Bikes do, this solves some problems of the other CVT's like 
friction ans 
slippage.
View the 
full article at 
LEGOISM web (Highly recommended site) and/or the video here, very instructive and well explained.