What does this "benchmark" actually test?

I apologize ahead of time if I posted to the wrong area...

My test:

time dd if=/dev/zero of=/dev/null bs=1024k count=1024

I was dealing with a "sluggish" machine awhile back and thought this one up. I found that it was a pretty good way for me to test if my "tuning" attempts had improved overall performance.

Now I find myself comparing machines... I can't stop!

Anyway, would anyone care to enlighten me on which components of the box are getting tested?

Thanks!
Steve

I only saw an increase in CPU activity when running that.