If you had a carbureted engine, it would (imperceptibly) lengthen the fuel runners and give the A/F tiny fractions of a second more time to atomize and warm up. A TBS would have to be REALLY long to have any perceptible effect, and it would then have negative effects in certain circumstances (i.e., when cold). In short, it's a waste of money for a carbureted engine save some VERY specific circumstances.
In a modern fuel injected setup, it does absolutely nothing save bilk snake oil believers from the carbureted days (and the people that listen to them) out of money. Since there's no fuel in the air until well after the TB, all it does (if anything at all) is reduce the air turbulence slightly causing a marginally poorer mixture. Some of the fuel goes unburned and causes the cat to die prematurely (again, only marginally). As far as any sort of perceived gains, they're just shiny placebos.
In short, it used to be snake oil and now it's just a waste of money. If you get a shiny one, it's one of (can't remember the redneck comic's name)'s idiot signs for your engine compartment.
