The negative (or concentric) part of any movement, is the lowering phase during the range of motion. For example, in the bench press, the negative motion would be the process of lowering the weight from the starting position to your chest (or immediately above). The negative portion of any movement is said to be the largest factor for hypertrophy (muscular growth). In doing negative repetions in a high intensity workout means all positive or eccentric repetions have been exhausted and you then prolong the set to also exhaust the negative or concentric part of the movement though the help of a partner or partners.