Black-hole X-ray transients (BHTs) often exhibit during their outbursts a characteristic q- shaped curve in a hardness-luminosity diagram. A rich phenomenology has been accumulated over the years regarding this diagram. It is desirable to have a physical picture of BHTs over the entire q-shaped curve, which hopefully will have predictive power. Such a physical picture is proposed here and it relies on two assumptions, easily justifiable. The first is that the mass- accretion rate to the black hole in a BHT outburst has a generic ?bell-shaped? form. This is guaranteed by the observational fact that all BHTs start their outburst and end it at the quiescent state, i.e., at very low accretion rates. The mass-accretion rate increases, reaches values close to the Eddington rate, and decreases again. The second assumption is that at low accretion rates the accretion disk is geometrically thick, ADAF-like, while at high accretion rates it is thin, Shakura-Sunyaev-type. This assumption has been verified by MHD simulations and it is generally accepted. Unlike phenomenological pictures typically invoked for BHTs, our physical picture explains the formation and the destruction of jets using the Cosmic Battery, as well as why BHTs traverse the q-shaped curve always in the counterclockwise direction and that no BHT is expected to ever traverse it in the clockwise direction. In addition, it explains the q-shaped curve with only one parameter, the accretion rate.