Atomistic simulations reveal a new and unique strengthening mechanism in nanotwinned metals governed by the collective motion of multiple necklace-like extended jogged dislocations. This mechanism prevails in a columnar-grained nanotwinned metal subject to an external stress parallel to the twin planes, provided the twin boundary spacing falls below a critical value. A theoretical model based on the de-pinning of unit jogs on twin planes is proposed to determine the flow stress associated with this deformation mechanism, and shown to be in agreement with atomistic simulations.