CYGNSS uses Delay Doppler Reflectometry to record ocean choppyness. What this means is, essentially, the satellites listen to GPS signals bounce off the ocean surface and look at how much the signal is scattered and delayed. In generalThe data is published at different levels, more scattering means higher choppiness and faster windspeedthe algorithms between the levels aren't necessarily public. However, although I believe there was some work done on fitting the function better than alevel 0 data has pretty usable data as is; for example, this article shows an example of how just simple linear curvesignal-to-noise ratio (listed as 'signal strength') gives an obvious difference between water and land.
Each satellite gets a few (3-4, normally) spectral tracks on the ocean where GPS signals bounce off; those are the spots we can measure. With 8 satellites in orbit we can get pretty good coverage of the east coast of the US with pretty fast turnaround times for data; the satellites are talked to nominally once per day but special circumstances (like hurricanes) occasionally call for more frequent downlinks.
Other answers, of course, covered all of this well, but I feel obligated to post an answer since I was a Flight Controller for CYGNSS :)