The Testo 770-3 Hook-Clamp Power Meter is a very unique hook style clamp meter that has many different features that help separate it from the traditional clamp meters you usually see on the market. The Testo 770-3’s most prominent characteristic is how exactly its hook retracts. The problem with traditional clamp meters is the hook opens outwards making it harder to use in tight spaces and increases the difficulty of grabbing onto wires.