  Steering is the entanglement-based quantum effect that embodies the "spooky
action at a distance" disliked by Einstein and scrutinized by Einstein,
Podolsky, and Rosen. Here we provide a necessary and sufficient
characterization of steering, based on a quantum information processing task:
the discrimination of branches in a quantum evolution, which we dub subchannel
discrimination. We prove that, for any bipartite steerable state, there are
instances of the quantum subchannel discrimination problem for which this state
allows a correct discrimination with strictly higher probability than in
absence of entanglement, even when measurements are restricted to local
measurements aided by one-way communication. On the other hand, unsteerable
states are useless in such conditions, even when entangled. We also prove that
the above steering advantage can be exactly quantified in terms of the steering
robustness, which is a natural measure of the steerability exhibited by the
state.
