An eccentric contraction is the motion of an active muscle while it is lengthening under load. …
An eccentric contraction is one of the three distinct phases in the movement of muscles and tendons, the other two are an isometric contraction (no movement), and a concentric contraction (contracting). …
Click PLAY or watch an example on YouTube.
That first drill is ideal for those cannot yet do chin-ups on their own.
Dave Tilley posted more on why he likes eccentric strength training.
Gymnastics Coaching.com 