Effectively, a focal reducer is like the opposite of a teleconverter.
https://en.wikipedia.org/wiki/Telecompressor A 2x teleconverter takes the central part of the image, such as 12x18mm on 35mm and magnifies it to cover the full 24x36 frame. As you know, the teleconverter increases the focal length and causes an effective speed loss. (In practice, the teleconverter intercepts the light rays before they reach the focal plane and rearranges them; this is a subtlety but is one of the reasons a teleconverter will work better with some lenses than others).
A focal reducer or telecompressor (or "speed booster") does the opposite, it takes the full frame and demagnifies it, scaling it down a little, like 0.8x, to fit on a smaller format. It shortens the focal length and so the effective lens speed increases. An 0.7x focal reducer will just about get full-frame 35mm onto an APS-C sensor, so these are/were used with small-sensor cameras when few people could afford a full-frame sensor, but there were a lot of 35mm lenses,. (I note that this is primarily a digital issue.)
Obviously, you can't demagnify lens coverage that wasn't there to begin with. I think the Hasselblad H is 6x4.5 (56x44mm) and X is 44x33cm, so it takes a larger lens and scales the image down. You couldn't take a 6x4.5 lens and demagnify its image onto a 6x4.5 piece of film and get more coverage for free; it would vignette, or the image would be terrible. Additionally, the combination has shorter back focus, which is why they tend to be used on mirrorless cameras.
"Speed boosters" usually seem to go to about 0.7x, I'm not sure there are common 0.5x demagnifiers corresponding to the ubiquitous 2x teleconverter. My guess is that poor optical performance and/or mechanical issues limit them to about 0.7x.
The obvious question is what happens if you stack a telecompressor and a teleconverter. I am not sure this has been tried. It will probably vignette a lot.
