Marinating the beef with bicarbonate of soda and cornflour is a Chinese 'secret' technique that has an amazing tenderising effect, giving a real melt-in-the-mouth texture. The other important factor with this recipe is to keep the heat high so everything starts to catch and char slightly - on a commercial wok burner this smoky flavour is known as 'wok hei' or 'breath of the wok', but you can also get pretty good results at home!