It was proposed that, at high velocities, extended dislocations became contracted because this lowered their kinetic energy enough to off-set the stacking-fault energy which limited their extension. When they were contracted, screw dislocations could cross-glide without the aid of thermal activation. This idea was consistent with the fact that cross-gliding was more frequent at low, than at high, temperatures. The critical velocity for contraction was equal to about one third of the velocity of shear waves.

Contraction of Extended Dislocations at High Speeds. J.J.Gilman: Materials Science and Engineering A, 2001, 319-321, 84-6