Instabilities produced by finite-resistivity effects in a plasma are of great interest in connection with research in fusion devices, solar flares, and geomagnetic substorms. We elucidate here the physical mechanism of this instability, and in particular, identify the tendencies in the system towards the instability and the tendencies opposing it, if any. As an illustration, we consider the example of the so-called gravitational interchange mode wherein a plasma with a statically stable vertical density gradient is situated in a vertical gravitational field and a sheared horizontal magnetic field. The physical picture developed here may be useful in sorting out phenomena that appear when more subtle properties of the resistive modes in a plasma are considered.