A whisk is a whisk, right? Wrong. Different whisks do different jobs, and the technique matters as much as the tool.
Three whisks, three jobs
Balloon whisk (round head): aerates. For whipped cream, egg whites, meringue, hollandaise. French whisk (narrower, longer): emulsifies. For vinaigrettes, pan sauces, gravies. Flat whisk (also called a roux whisk): scrapes and stirs against a pan bottom without spilling. For deglazing, making roux.
The wrist motion
For aeration, hold the handle at the top and use a big circular sweep from the wrist. This whips air in. For emulsification, hold lower on the handle and use a quicker, tighter figure-eight motion. This forces the mixture through the wires.
Common mistakes
Bowl too small: everything splashes out. Whisk too small: takes forever. Cold ingredients when they should be room temperature (whipped cream aerates faster when cold; egg whites aerate faster when at room temperature). Bang the whisk against the bowl to shake off batter: bends the wires. Just scrape with a spatula.