A roundup of explainers – choose your flavour.
Focal Reducers / Speed Boosters | Explained (via CVP):
How Crop Factor Affects Focal Reducers & Macro Reproduction (via Gerald Undone):
The Effects of Sensor Size and Foal Reducers (via Anamorphic on a Budget):
Using different combinations of various lens sizes, focal reducers, and image cropping, Devan Scott demonstrates why the excitement over large format cameras has more to do with marketing than with the actual math:
Further Viewing
A related, but separate concept is aspect ratio: