Thought I'd share this with you. This is used to thicken the sauce a little in a stir fry, so that its not too runny on the plate when served. In Cantonese, we call this thickener 'heen'.
Normal method: potato starch cold water, add a little soya sauce and sesame oil. Add mixture whilst dish is simmering a little at a time to ensure correct thickness of sauce.
Glass method: instead of soya sauce, you substitute it with salt, and use cornflour instead of potato starch (potato starch tends to give the sauce an opaque effect). This will make you serving gleem that extra amount, with the colours of the dish unaffected by the thickener. If you want you sweet and sour dish to look vibrant, you greens stand out more, or just a dish with very easy colours, this is the method to go for.