This is one of those tools that, once you have it, you can't remember how you managed without it. The shoe tuner is designed as a reference allowing the brake pads to be correctly aligned with the rim, thus improving stopping prowess and eliminating annoying brake squeal.
It seems compatible with most types of rim brake and can be adjusted for the narrowest of road rims or big, beefy downhill designs - great if you have road and mountain bikes in your stable.
Open the brake's quick release a little to allow the guide to pass through the blocks and clamp gently into place using the small plastic wing nut. A small elastomer-type cartilage sandwiched between the arms gives the tool a progressive and tactile feel.
Once in place, close the calliper's quick release and align the blocks as required. The cynical among us would point out that £13 is a lot to pay for an alignment tool and that using strips of card against the rims is as good a gauge as any. However, this tool allows very accurate adjustment in a fraction of the time, which is a worthy consideration for busy workshops or riders with a fleet of machines.