I've seen this type of boost method before, and if it is the same, then its simply turning off any optimisation your graphics card is doing.
Can only speak about ATI as its what I own, but I'd imagine Nvidia have similar.
When you rename the main exe you are simply fooling the Graphics Card Drivers into thinking its not the game in question, therefore the optimisations are not loaded. This may work for some games, but you may also notice they don't look as nice, hence the apparent boost in FPS. This may also account for the less laggy feel, as it sounds as though you were are getting screen lag, not net lag. Test it out, take a screen shot with it renamed and the original, chances are you'll notice differences.
Turning off AI in the driver controls will have the same effect, as this is what renaming it effectively does.
Treid it on F.E.A.R and no change, so its by no means a wonder cure for boosted gameplay. Personally I prefer to tweak games myself
Plenty of options in game and in the Graphics Card Control Panel to make any game run a little better.