It's in the applications guide under bi-linear restraints (page 3-41) and if you hit F1(help) when inputing the K2 parameter.
I quote from the CII applications guide:
'Both the yield stiffness (K2) and the yield load (Fy) are required entries. The initial stiffness (K1) may be left blank, and a
rigid initial stiffness assumed. The yield stiffness may be negative if necessary. Some subsea pipeline resistance tests have
shown that load carrying capacity drops after the “ultimate” load is reached, and displacement continues.'
And, no, a negative yield stiffness would mean that the stiffness reduces once the yield force was reached, not that it would start to move the element in the opposite direction to force application in some way.
Thanks for the help so far.
Matt