What does it mean to 'express' a citrus peel?

Expressing a peel means squeezing it over the cocktail to release the citrus oils onto the surface, where they add aroma. Hold the peel between thumb and finger, skin-side down toward the drink, and pinch sharply.

Citrus peels carry aromatic oils in the skin; the rest of the fruit holds the juice. Expressing the peel separates the aromatic component from the juice component. Technique: cut a wide strip of peel (lemon, orange, grapefruit; avoid the bitter white pith underneath as much as possible). Hold it skin-side down about an inch above the cocktail surface, between thumb and middle finger. Pinch the peel sharply — twist outward slightly — so the skin folds and the oils mist out as a fine, briefly visible spray. The oils land on the surface of the drink and on the rim of the glass. You'll smell them immediately. Either drop the peel into the drink (most common) or rim the glass with it first, then discard or drop in. The aroma layer the expressed oils add is significant — sniff the cocktail before and after, the difference is dramatic. For drinks that depend on this (Old Fashioned, Negroni, White Russian variations, most Martinis with lemon twist), do not skip this step. Without it, the same drink tastes one-dimensional.