its a bit of a balancing act to get it to brake, but it can. i have the line go back to the cockpit
to an unused winch
, and can tension it there, as well as the knob on the unit. I primarily use the tension at the winch to control the friction.
I have it down to 3 settings - loose=swings, tight=brake, supertight=preventer mode.
If I take the sheet out of the winch and loosen it, when its locked up, it will start to 'brake', slow to fast, depending on how much I let out the sheet. Doesn't take much (talking an inch or 2 maybe) to get it to brake.
The manual shows a block and tackle set up to tension the line (IIRC), I use a spare winch, which seems to work.
It takes some practice to get it 'just right'. It is a combination of knob tightness and line tightness.
When you get used to it, you will wonder how you ever sailed without it!