Combine the yoghurt, cumin and salt in a deep bowl and beat vigorously with a whisk or spoon until the mixture is smooth. Beating constantly, pour in the ice water in a slow, thin stream and continue to beat until all the ingredients are well blended.
Taste for seasoning. (A quicker, though hardly traditional, method is to combine the yoghurt, cumin, salt and ice water in the jar of a blender and blend on high speed for 1 minute, or until the mixture is smooth.).
Pour the lassi into a chilled pitcher and serve it from chilled tumblers. If you like, you may prepare the drink ahead of time and keep it chilled in the refrigerator until ready to serve.