Before revision:  When the amino acid mutation of S protein causes a substantial reduction of elastic energy the equilibrium is biased to the open conformation.
After revision:  The former is related to the change of affinity of RBD to ACE 2 due to the mutation in the subdomain RBM (receptor binding motif) of RBD. The latter is caused by the change of elastic energy of S protein. When the affinity has increased significantly and/or the elastic energy has been reduced substantially the equilibrium is biased to the open conformation.