What is the theory behind low voltage focus CRTs?

[LukeSimon]

What is the theory behind how low voltage focus CRTs work, and how does that compare and contrast to the high voltage focus CRTs?

I know that in both low voltage focus and high voltage focus CRTs, the G1 control grid uses a low voltage (relative to the cathodes) to force the cathode ray to squeeze through the G1 control grid's aperture, which in a CRT is a metal cylinder surrounding the cathode with a small aperture at the exiting end. This is an example of how low voltage can be used to make the cathode ray a thinner, sharper beam. Do low voltage focus CRTs use a similar mechanism? What are good articles to read on this topic?