Continuous Tension;

For a muscle to receive full benefit from an exercise,the weight used should continually be felt along the fullest possible range of motion. Unfortunately, many exercises do not allow for stress to be placed on a working muscle at all times. That is where the continuous tension principle comes into play. Moving the weight slowly and in a purposeful manner and not allowing the weight to hang at the bottom, as in the standing barbell curl, for biceps, or not allowing the weight to sit against your chest at the top of the curl, therefore keeping continuous tension on the muscles throughout the whole exercise. Thereby recruiting every muscle fiber possible throughout the set and stimulating more muscle growth in that muscle.
Another mistake i see many inexperienced lifters doing is locking out at the top of barbell bench presses, be it flat, incline or decline. Locking the elbows even for a second at the top of any bench press movement will not only put undue, unneeded stress on the elbow joints, inviting injury on the joints. Not only is it bad for the joints but it negates the purpose of really stressing the pecs and fails to recruit as many fibers as would be fired if the elbows were not locked out at all.
Keep in mind this principle is to be used when getting ready for a show or when you change up your lifting routine from a winter schedule to a summer one, where lighter weights and better form is needed. In other words, it is used for more sculpting the muscles although it is possible to build muscle when used properly and safely with enough stress placed on the muscle. Even though you may not be using maximum poundages, a muscle can still be stimulated by placing continuous tension on it. Those seasoned lifters know what i mean by this and have probably been cycling their workouts for their entire lives, i am just trying to get the grey matter upstairs working and help you remember some principles that you may have forgotten!