You need to place the wrench around the part that you need to manipulate. Its tooth then fits into the part’s notch, such as in the case of a crimped lock nut or a bevelled pipe fitting. As you turn the wrench, the tooth prevents the head from slipping out. It allows you to fix or adjust the part quickly. The basic function of a hook wrench is to tighten, loosen, or hold round but also notched components such as lock nuts and pipe fittings. Apart from that, you can use them pretty much anywhere you find nuts with holes or notches.