You can look at pendulums in two ways really. It can many times depends on your use of them though both fields use the subconscious energy. Some have used them as a tool of scrying (focusing understanding from the subconscious through an object or finding "inner wisdom") , or dowsing (finding information of the physical world through using and focusing earth energies in vibration to your own). If used in scrying, the pendulum is like any other object you use in scrying, it focuses your energy of what you desire or seek, allowing the vibrations your subconscious to physically shows a response through moving the pendulum via your own bodies vibrations). In other words, If you understand what scrying is, you understand what a pendulum actually does. It carried and physically shows your bodies vibrations through moving the pendulum. The weighted object on a string is actually for a reason. In order for your vibrations to show, their must be a medium light enough to be moved from your bodies even smallest vibrations. A string or chain is a straight (due to the weight keeping it so on the end) and light material. The weight at the end is able to keep the string/chain from simply shaking but rather moving in large enough movements to be more direct and obvious. It's also important to understand that your subconscious carries vibrations that many of us aren't even aware of. Your subconscious is constantly taking in information and remembers it from not only your experiences but from everything around and in ether. That is why scrying is such a useful tool and why pendulums have been such a long respected medium. The mediums use goes all the way back to the ancient Egyptians and Chinese or further back (not necessarily in scrying but rather dowsing), so you know the tool and it's form are well respected for a reason.
Now dowsing... As stated above, dowsing is focusing earth's energy such as ley lines, rocks, plants to seek/find things such as water or direction or even lost objects. Some do believe at the same time that the information still comes from the person's subconscious instead of the earth. Others believe both are the case but it's really your decision of what you think is correct cus either way the pendulum will still work. If you have any questions about them, email me.
Also, a good book for beginners I have used and learned about through others is "The Art of the Pendulum" by Cassandra Eason. It's direct and simple but enough information to fully understand without getting too far into detail.