  We present a unified semi-quantitative model for the disc-jet coupling in
black hole X-ray binary systems. We argue that during the rising phase of a
black hole transient outburst the steady jet known to be associated with the
canonical 'low/hard' state persists while the X-ray spectrum initially softens.
Subsequently, the jet becomes unstable and an optically thin radio outburst is
always associated with the soft X-ray peak at the end of this phase of
softening. This peak corresponds to a 'soft very high state' or 'steep power
law' state. Softer X-ray states are not associated with 'core' radio emission.
We further demonstrate quantitatively that the transient jets associated with
these optically thin events are considerably more relativistic than those in
the 'low/hard' X-ray state. This in turn implies that as the disc makes its
collapse inwards the jet Lorentz factor rapidly increases, resulting in an
internal shock in the outflow, which is the cause of the observed optically
thin radio emission. In addition, we estimate the jet power for a number of
such transient events as a function of X-ray luminosity, and find them to be
comparable to an extrapolation of the functions estimated for the 'low/hard'
state jets. Finally, we attempt to fit these results together into a coherent
semi-quantitative model for the disc-jet coupling in all black hole X-ray
binary systems (abridged).
