I don't know what it's really called, but they're the 2 black end pieces that came with the S-model handlebar.
I removed them when I switched to a ProTaper handlebar, thinking it's too much of weight. Then I came to know that they're supposed to be some kind of balancer or stabiliser and put'em back on (Modified to fit the ProTaper). The difference is, with the weight, the bike feels so much smoother thru the bars! The comfort is worth the extra weight and added width.

Then again, I might be the only clown that didn't know that in the first place. If so, please ignore this.

I removed mine also to fit the Rally Pro Guards. I did not notice any real difference in the steering. Those are heavy little bar ends though. Chris

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DRZ-400S

Oops, sorry. I actually meant the vibrations, though it's not quite a problem when this bike is concerned.
That heavy thingies somehow reduced the vibration that goes to the hands (w/o gloves).