In order for wires to go live, they need to be attached to an electrical current. This is where a compression tool comes in. Compression tools have a slot to hold the compression connector, so when you insert the wire, you can press down and clamp it around the wire. There are a few different styles depending on what kind of electrical wire you're trying to connect. A ratchet style tool works like a ratchet wrench with different levels of tightness that can be adjusted, and a lever style is great for smaller compression connectors.