Muscles exert force concentrically as they shorten and eccentrically as they lengthen. Jonathan Mike from the University of New Mexico, and co-workers, described the benefits of eccentric contractions for building muscle size and strength. Muscles can exert more force during eccentric than concentric contractions, which stimulates greater degrees of muscle hypertrophy. Also, muscle activation and tension is greater during high-speed eccentric contractions than slow contractions.
Athletes typically incorporate eccentric training without even knowing it: their muscles contract (eccentrically) when they lower the weight during bench presses, squats and curls. They could incorporate eccentrics further by increased emphasis on the down portion of the lift. For example, using a power rack, they could do heavy singles on the down portions of bench presses and squats using more weight than their one-repetition maximum. During kettlebell swings, a partner could increase the eccentric part of the exercise by pushing on the kettlebell during the down portion of the exercise.
People have long used negatives to accentuate the force used in eccentric contractions. Have a spotter help you during the concentric phase of the lift, and lower the weight by yourself during the eccentric phase. As discussed, do eccentric exercises at high speeds for the greatest load. (Strength and Conditioning Journal 37, (1): 5-17, 2015)