Vic Dura wrote:
I'm just curious as to what the theory behind that is.

When the refrigerant charge is low the evaporator coil gets starved for
refrigerant and this results in reduced pressure at the inlet piston or
expansion valve, thus allowing the refrigerant to vaporize at a lower
temperature - below 32 degrees. At this point the first part of the
coil will freeze. Then, since ice is a fairly good insulator the
refrigerant will now travel further through the coil before encountering
an exposed surface. More ice forms and the process continues. Gradually
most or all of the evaporator coil will be covered with ice. This of
course blocks air flow through the coil.