The link doesn't work, but I think I know what you're talking about. Instead of a curved lever like normal, they are more like a little knob that goes straight from the bar that you press down to apply the brakes? If those are the ones, I remember reading about them not being called "brakes" so much as "speed modulators" because they don't actually stop your bike so much as slow you down a little for a turn. If that's the case, I would never use them on any course that had a hill.

Right...Hed makes one of these as well, called the "3rd brake lever." Designed to scrub speed for a turn so you don't have to come out of your tuck while cornering; not meant to replace your regular brakes.