Taken from Variables for Weight Lifting and Exercise © 2003 Anthony Sell - All Rights Reserved
The presence of a sensitive focused and experienced spotter is paramount to real progress. This becomes readily apparent during intense lifting sessions, where the focus is on reaching the point of muscle failure. This is especially true when performing exercises which include movements that suspend the weight over the body. Injuries are not conducive to progress, avoid them at all costs by knowing your limits and using a spotter.
It is important for training partners to learn exactly what is safe and acceptable form for spotting before trying to spot for each other. There are certain guidelines that are helpful to follow so that the spotter will not impede the progress of the athlete they are spotting.
When the spotter aids with the repetition, it is to prevent the weight from falling on the athlete, or to maintain a safe action or path of motion that will not risk injury. This means reinforcing the stabilizing muscles when they give out, making sure that the action of the exercise is stable and consistent. This also means aiding in the last two or three repetitions, by adding just enough support to barely keep the weight moving.
At the end of the set, the spotter should ensure that the weight is put back on the bench supports without incident, as that is when the athlete is prone to collapse.
Last, and most important is the spotter's responsibility to be aware of the athlete's state of mind. This includes concentrated attention toward signs of fatigue and lack of motivation. Motivation is one of the most important factors in weightlifting; it is what makes the difficult possible. It is the spotter's responsibility to keep the athlete motivated through the set, then offer advice after it.