In my limited experience with mainlining (this is my second time doing it), I would have to say that it's not so much a yield modifier as it is a consistent bud site modifier. You don't necessarily get more buds, in fact one could argue that if you vegged a photoperiod for the same amount of time naturally before flowering you would get more bud sites. But what it does do is puts each of the main bud sites an equal distance up the stalk, so in theory they all get the same cut of the nutrient mix. So instead of the one really nice dense main cola, a few almost as dense side branch colas, and so forth on down to the little fluffy popcorn bud sites way down near the bottom you get a nice even canopy with all the bud sites taking relatively the same effort on the plants part to get nutrients to them and therefore getting an even mix.
This was the one I chopped last August. All eight of the colas were a nice, dense bud structure and consistent around the plant.