The way I have always thought of it is fire needs fuel, air and heat (ie; spark). At a certain level these give the biggest flame (therefore hottest and most powerful). Any change to fuel or air (you can't change spark in flight) from this certain level will result in a cooler, less powerful and more 'wasteful' flame.
By wasteful I mean something won't be totally burnt at ROP this is fuel and therefore 'dirty' and LOP this is air therefore 'clean'.
Is this a fairly accurate simple summary?