Post from 'Strength and Conditioning Research' Facebook:

One of the key ways in which eccentric training produces specific gains in eccentric strength is through an increase in neural drive to the muscle.

In untrained people, the ability to voluntarily activate a muscle during an eccentric contraction is much lower than in an isometric or concentric contraction. With training, we improve our ability to perform eccentric contractions.

Current research suggests that untrained people find it harder to activate muscles during eccentric contractions because of inhibition at the spinal level. Training reduces this inhibition, and allows us to express a higher level of force.