for the onset of

Can anybody explain what 'for the onset of' means here. Never mind the scientific terms:

In most cases the zero-shear viscosity (ho) depends primarily upon the average molecular weight and not the molecular weight distribution. The shape of the viscosity curve, on the other hand, is a function of the MWD of the polymer. The magnitude of the critical shear rate for the onset of non-Newtonian behavior is determined by the MWD. Thus, two polymers that have similar MWs
but different MWDs (narrow versus broad) will have quite different flow curves.

Here, "for the onset of X" means something like "in order for X to begin" or "necessary for X to begin".

The magnitude of the critical shear rate necessary for non-Newtonian behavior to begin is determined by the MWD.

Thanks