I am reading the Abaqus documentation (https://help.3ds.com/2021/english/dssimulia_established/SIMACAEGSARefMap/simagsa-c-ctmreduced.htm?contextscope=all&id=edeb6f07bacb4bee8bd759fafe34c490) to solve an hourglassing issue in my model. The explanation of shear-locking and why it happens makes sense physically, but I am having some difficulty understanding what is meant by a "zero-energy mode" and why does it not have a stiffness that makes it unable to resist deformation?

Abaqus Explicit Abaqus Abaqus CAE