is an exercise where the movement tends to increase the length of the muscle, whilst the muscle is under tension. This appears to particularly trigger hypertrophy
in muscles concombinant with increased strength.
One theory about this says that the proteins in the muscle come unzipped, particularly during this type of exercise and end up damaged as they try to rezip back together. This leads to Delayed Onset Muscular Soreness, and temporary reduction in strength for a few days. (Interestingly, exercise that does not involve eccentric movements rarely cause this.) During the 11 day repair process the proteins get replaced, and more are added on top; presumably the muscle is trying to prevent this occuring again. The extra proteins result in hypertrophy and increased maximum strength.
Experiments show that this exercise gives strength increases that reappear even years later when exercise is reestablished.