A rotating jaw vise is a vise with multiple jaw types that can be rotated around the x axis. This usually means the vise has a pipe jaw, and a more traditional vise jaw. Rotating the jaws is usually pretty simple.
Some vises, like my yost750-di vise, have a pin that you pull to unlock the heads rotation abilities. After pulling the pin, push the vise head to the angle you want. Once you have the vise head at the angle you want, simply let go of the pin and it will re-lock once it finds a preset position. It is important to make sure that your vise is locked in place, you don’t want it to wiggle around once you start working on your work piece as this could be dangerous for both you and your equipment. It usually makes a large clicking sound when the pin slides into place and locks the vise head.
Other vises have a handle you that pull to unlock and lock the vise head.
It’s honestly pretty straight forward, most vises will come with an instruction manual when purchased new that will give more details. It’s hard for me to provide specifics when there are so many different models of rotating vises.
If you are buying used, you can usually find the owner’s manual or additional materials such as an isometric view that will help you figure out how to rotate the jaws on your vise.