Mountain Biking - Chain tensioner?

I saw this mountain bike magazine and saw this rider on his bike zipping through the trails. On his bike, he had this chain tensioner? Near his chain rings. It looks like its coming from the bottom to make sure the chain doesn't wiggle severely when going fast on the bumpy trails. Does anyone know if that's the correct term and where to get one?

Chain guide. Types abound for one-, two- or three-chainring applications. Examples here:

http://www.blackspire.com/qs/category/83/5947/0/0

Also check out MRP for similar chain guides.

thank you so much guys!! I was really frustrated with this bike with no front derailleur and chain popping off everytime I enter a bumpy section. Now do most people do this with only single chain ring? i want to put this on my other mountain bike and I its a 24 speed. The bike only having 1 chainring, I don't think it could go through 8 of the gears in back with out cross chaining. How do racers come about doing this with out having extra gearing you don't need?

Also check out MRP for similar chain guides.I was merely pointing out examples, as Blackspire has a wide range of styles; I wasn't trying to give him a shopping guide at the time :rolleyes:. Carlo, there are so many brands it would make you dizzy.

In answer to your next question, they are usually used with one or two chainrings and often a bashguard too. On the single ring setup, the ring is in the middle position so there aren't cross-chaining issues.

thank you very much dminor! you helped me out greatly! I went with the e13 lg1+, I should have went with the ls1+ because of cost, but sometimes I don't think :/