Not necessarily TT specific... some people do run them on regular road bikes. It is a Ceramicspeed OSPS (over-size pulley system). The pulley wheels themselves spin on ceramic speed bearings, so there's very, very little drag. The reason they are oversize is because forcing the chain to bend more tightly around smaller pulleys is not as efficient. TT's are all bout being as efficient and fast as you can, with the power you have.
If you watch the professional racers that specialize in TT's, you'll see that many of them will often run a 55 or 56 tooth chainring... most people, incorrectly, assume that that they do this to get more top end speed (a 55x11 combo will be faster than a 53x11 combo, assuming they've got the power to turn that gear) but actually, this isn't why they do it. They're choosing the bigger chainrings because they've researched the course and know that the 55 tooth chainring will keep them in the 15 or 16 tooth cog on the cassette for as much of the course as possible. This actually is more efficient in 2 ways. 1. It means they have a straighter chain line... that angled bend in the chain is eating up precious watts. 2. The 55x16 combo means that the chain isn't bending as tightly as it would if the rider was riding a 53x13, saving more watts.
This is all pretty nit-picky stuff for sure, but that's the name of the game for TT's... its about being aware of all of the little things that save speed and make you go as fast as possible.
