A simple model of the development to large amplitude of the m=1 (helical) mode in the Z pinch is described. It is shown that the large inductance changes associated with this instability, and the interaction between the pinch and the external circuit play crucial roles in the phenomenon. A criterion for the 'most unstable' wavelength is given. The absence of long wavelength modes, and the origin of the rapid expansion, observed in this type of instability, are explained