Each different weight training exercise is used only for a certain muscle area or group of muscles. This muscle area is the "primary" muscle area for that certain exercise or motion. After several reps in a set, the motion will become difficult. People will then sometimes try to use other muscle areas to help the primary muscle area with the motion by jerking or making a small change in the motion. This is bad form. The reason this may hurt somebody is that these other areas may not be strong enough to move the same weight as the primary area, and they may be strained if used.