They're explained in the appendix of the manual. If you have seen Maya's driven keys, this is pretty much the same idea. Where the other ERC functions give you a simple linear relation between two parms ('as parm A increases, increase parm B too'), the new valueOpKey allows you to establish more complex relationships based on key positions ('as A goes from 0 to 4, set B to 90 to -3; as A goes from 4 to 10, set B to -3 to 40').