What technique should be used to layer drinks effectively?

To layer drinks effectively, using a bar spoon is the ideal technique. This method allows for precise control when pouring liquids of different densities. By employing the bar spoon, the bartender can gently pour the liquid over the back of the spoon, which helps to create distinct layers instead of mixing them together. This is particularly important when working with ingredients that have varying densities, as it prevents them from blending and maintains the visual appeal of the layered drink.

The other techniques mentioned do not support layering. Shaking and stirring typically combines ingredients, resulting in a homogeneous mixture rather than distinct layers. Pouring swiftly could create splashes or cause the liquids to mix, also detracting from the layering effect. Mixing vigorously is the opposite of what is needed; it will ensure that the drink components are combined thoroughly rather than kept separate. Thus, using a bar spoon is the best method for achieving a beautiful and visually striking layered drink.