It alters the take-up point with the same effect as the cable adjuster on the handlebar has.Wind in the latter so that it's pretty much at the start of its range (also has the benefit of reducing the exposure of the slot so less rain and dust gets into the cable). Then at the bottom, with the locknut slackened off, turn the screw in until the take-up point is near where you like it to be. Tighten up the locknut, bung the bung back in. Fine-tune at the top end of the cable.