Hold the peel skin-side down over the glass. Squeeze and twist firmly to express the oils onto the drink's surface. You should see a fine mist if you're hitting it right. Then either rim the glass with the peel, drop it in, or discard. Expressing is for the aroma, not the taste — the oil is what hits the guest's nose first. Discard if the drink is dry-style (Sazerac, classic Martini). Drop in if the drink benefits from the peel's presence (Manhattan, Negroni).