Ever do any trail running? Huffing and puffing at low speed going up? Running like superman downhill with a smile on your face? Eccentric cycling is like running downhill. Concentric is like running uphill.
When you run uphill you do work just like you do when cycling. When you run downhill you absorb work and only perform a braking action. That's the same as eccentric cycling. If you happen to be in Salt Lake City drop by my lab and try it out.
Does that help?