Zog The Undeniable wrote:

I have a pair sitting around in the spares box. I never thought they
had any more power than regular brakes when I used them (they came with
a bike).

Anyway, there's a helix in there which applies the brake harder as the
pads are dragged forward by the rim.

I have a pair made by Sun Tour - they were only for back brakes I think.
I replaced with V brakes which work better, although my old U-Brake on
my Muddy Fox is pretty good too.