The most people who have bad form in this exercise will push with the feet to make their back bent and lifted up off the bench. This moves up the stomach area over the chest and helps the lower chest muscles move the weight. This may hurt the low part of the back.
Because the weight is directly above the body, a "spotter" or helper is used by many people in this exercise. The helper will stand behind the head of the person doing the exercise. If after doing some of the exercise the person doing the exercise cannot move the weight, the "spotter" will help the person move the weight back up to the starting position by taking hold of the bar and lifting together with the person.