strain rate dependent (Abaqus)

Hello !!!

I try to model the softening  of the masonry resistance in time.
For that I have tested the Cowper-Symonds law that exists in Abaqus. but I can not understand until now the effect of this law sometimes seems to me that it reduces déformations. sometimes it affects the stress (decrease or increase). how this law can modifie the stresses?

if you have any details about Cowper-Symonds law please to answer me

Thanks