what is this (pics)
 

came with my dura-ace 10 speed shifters, can't figure out what it is slash how to use it

It is an inline "gear indicator" that will tell you the chain position on your rear cassette. Mine is positioned a few inches from the right brifter on the RD cable. There is a small red ball inside the clear tube that moves back and forth on the cable to your rear derailleur. :thumb:

snake flashlight, or new fangled mtb shock pump.

actually Old School is right.

It's not needed.

When I switched from my mountain bike shimano setup with a thumb shifter to the "new" 9 speed Jamis Nova, I complained to my lbs that I missed the visual que of the thumb shifter. They installed one of those inline gear indicators.

As I got used to the 9 speed I realized that all you have to do is look at the gears. In any case, the plastic starts getting UV damage and it gets harder and harder to see anything. And when I recabled I just left it off.

It is a good visual aide for newbies learning how to use the new gears.

That is one awesome key fob waiting to happen.

I had some for my XTR DC shifters and thought, boy these are stupid so I never put them on.
I just got a road bike with Dura Ace shifters and this was already installed, and I really like it.
I always looked down at the rear derailleur to see what gear I was in, but find this way better. Old habits die hard and I still find myself looking at the cassette, but less and less.

I've got one on one of my road bikes, but I really appreciate it most on my tandem. Don't have to ask stoker what gear we're in and know when to trim and such based on indicator.

I have one of these on each road bike that my wife and I own and one on the tandem. A bike is not complete without one of them. 9 and 10-speed versions are easily available from local bike stores, and sometimes they'll have an old 8-speed version lying in the back of a drawer.

A lot of people who care more about fashion than function say that you don't need one - but life is certainly easier with one, and it weighs less than 5 grams, so why not?

I find it very odd that it comes as standard part with Dura Ace brifters, and I believe there is a similar thing integrated into the top of the current Sora and Tiagra brifters, but people who buy 105 or Ultegra brifters get nothing - we have to buy the Dura Ace model from the LBS (very few online stores carry them).

I recently started piloting for a blind rider, and it wasn't until then that I noticed how often I peek between my legs to check which gear I'm in. :o