The CT clamp reads the current without having to be directly connected to the wire. It just 'clamps' onto the leg and is non invasive. https://en.wikipedia.org/wiki/Current_clamp
Here's one of my panels with the clamps on. The top clamps read total power, the individual clamps below are reading one leg each. for a 240v you need both since each leg is only supplying 120. My dryer also sometimes only uses 120v when it's not actively heating.
