http://www.cardplayer.com/cardplayer...s-up-with-lags
It's a good article - the 'meat' of why (some) LAGs are so good is about 2/3 of the way down the page:
As most of us know, an EV graph for preflop hands follows an "inverse exponential curve": AA >> KK >> QQ >> ... BUT the EV curve levels-out very quickly as you progress to 'mediocre hands' (if you want to graph this, there are sites that provide the data).
The 'secret' is, preflop, good LAGs play most mediocre non-denominated hands the same way because the EV difference between these hands is miniscule (both of them can even be -EV).
But with good postflop play, the hands become basically equivalent.
So good LAGs get more postflop play because they realize many mediocre hands are "EV equivalent", whereas the non-LAG would be reticent to play some non-traditional hands. Miller gives some examples.