I think those are actually just paul track ends that you have to get built into a frame.

http://www.paulcomp.com/drop.html

I could be wrong.

They seem like they would be annoying, as you'd have to either use a master link in your chain or back out the screws to loosen your chain enough to get it off the cog/chainring.

The paul style track ends and the chain tensioners that you attach to the dropouts are both doing the same thing; the paul is pushing the axle back, and the chain tensioners are pulling the axle back. Both lead to a tighter chain as you turn the screws