The chain tugs prevent the wheel from slipping forward, i.e. which would make the chain looser, not tighter. They are tension decrease preventers. Take a look at a pic of a chain tug not mounted to a bike. The ring goes around the axle with the bar flush with the back of the track end. the screw pulls the ring (and axle with it) towards the back of the track end. Now the back wheel can't side forward as the force of pedaling, skidding, etc would cause, because the axle is held a set distance from the back of the track end.
That said, assuming your bolts are tight, there really isn't much of a use for a tensioner as a 'tension decrease preventer'. Some people find they make adjusting chain tension easier or provide bottle opening abilities though.

ah, I was not picturing the mechanism correctly. I see how it might be useful when removing the wheel, the rings would put the axle right back where it was.

Now that I see it the right way around, though, I wonder how much additional help two little bolts are going to be when you already have big 15mm nuts on a big axle.

thanks strongbad, I mean bombusben

I think you might be confusing the limit screws found on older road bike dropouts with tensioners used on BMX and track bikes, where the dropouts open rearward. By pulling the axle of the rear wheel away from the front chainring with the tensioner (whose set screws are braced against the end of the dropout) you are able to set/maintain a desired tension on the chain that will not be compromised by loading the drivetrain thru braking, skidding, or just riding hard.

...better late than never.

Bingo, But you should be able to buy them from Danscomp.com or any other good BMX site. Redline doesn't have them on their website, but they exist!

Do they make them for road frame/conversion fixies? If so, where do I get those?

As far as I know, no. They would basically have to work backwards - pushing instead of pulling - for that to work. I would imagine it's probably even less of an issue on conversions than on track bikes just due to the nature of the horizontal drop outs.

I splurged on some MKS NJS tensioners today (only $5 more than the BMX ones I had before). Mounted them on my IRO. They fit perfectly! And, oh my god, are they good looking, nice and small and easy to deal with. I had those ManOwar two screw tensioners before and boy did they suck. Anyway, if they fit your frame they are sweet.