Shaking a cocktail rather than stirring it will both blend the ingredients together and chill it, if you’re shaking it with ice cubes. Also, a good rule of thumb is that any cocktail that includes citrus should be shaken not stirred. The reason for this is shaking it livens up the lemon or lime flavour and you get a better end result. For a sour recipe, such as this vodka sour or a whiskey sour, the egg white is in there to create a white foam on top. For this reason, give it at least 30 seconds of shaking to incorporate tiny air bubbles in the mix, so your fruity cocktails will have that nice white foam topping.