Compound bows are designed to reduce the force required to hold the string at full draw, hence allowing the archer more time to aim with less muscular stress. Most compound designs use cams or elliptical wheels on the ends of the limbs to achieve this.
Compound bows are engineered to reduce the tension required to hold the bowstring taut, allowing archers more time to focus their aim and minimize muscle fatigue.  Most modern compound bows utilize cam systems or elliptical wheels at the ends of their limbs to accomplish this.