A z-pinch squeezes a column of plasma with its own magnetic field. Squeeze it hard enough and the column does not stay a column: it pinches into a chain of bulges and necks, like a sausage string. That is the m=0 instability, and it is one of the oldest obstacles to confining plasma for fusion.

The figure is a simulation of that instability developing in a cylindrical equilibrium. It was the first figure et al. ever printed.