Any time you flex a spring you cause it to store energy. If you take a coil spring and compress it, you have put energy in. If you release the compression, the spring will straighten back out and give up the energy. It’s the same with the limbs on a bow. They are simply springs (cantilever springs). With a compound bow, the limb tips aren't flexed backward toward the shooter by the string like they are with a traditional recurve that has no cams. Instead, the string and harnesses pull the limb tips toward each other. Here’s how a compound bow works.
UNDERSTANDING THE ECCENTRIC SYSTEM
Compound bow eccentrics are simply a system of levers designed to give you a mechanical advantage when storing energy in the bow during the draw cycle.
The basic eccentric system on a compound is made up of a string, one or two eccentrics (or cams) and one or two harnesses or cables. (A modern single-cam uses only one cam and a power cable.) With a traditional two-cam bow, one end of the harness attaches to a peg on the cam. The other end is attached to the axle on the opposite limb tip. When you draw the string, you pull the eccentrics around and that wraps the harnesses up and pulls the limb tips toward each other. Two-cam bows have two harnesses that work together to flex the limbs.
Single-cam bows, on the other hand, have only one harness that is used to pull the limb tips toward each other. When you draw the string, the cam turns and does two things at once. It lets the string out on the front and back while at the same time wrapping up the power cable to flex the limbs. The result is the same, the limb tips bend toward each other and the bow stores energy.
Read more about Understanding Compound Bows.