they work after a fashion but aren't scientifically accurate. how you would use one is think, just your wood, your conditions. OK, say you have a starting point on the interior of a fresh cut redoak, you might hit almost 40% on what it reads. Once after it is cut split and stacked, and you resplit a larger split and test the interior, if it reads below 20, ready to burn.
All species read different, other factors (all sorts of other factors) come into play, so don't regard your readings as highly accurate, but just as a reasonable ballpark figure.
I find them fun to use. They aren't necessary, but for 20 bucks, to check the status of your stacks, no harm. Once you have developed a feel for the wood and your own local drying conditions, you can correlate what readings you get with the normal criteria of cracks in the wood, weight loss, a graying color, and the universal clank test, slamming two splits together and seeing how the pitch changes.
Once you have a good, well cracked, high pitched "clank" piece of wood, split it, see what it reads. You will now have a low/high reading between fresh cut green and ready to rock.
If I can be loose with these auditory representations, new green wood thunks, a dull lower pitched sound, it eventually changes to the higher pitched "clank", well described before as the sound of two baseball bats clanking together. That's ideal.
My 2 year + old well dried oak heartwood reads mid teens average on my meter. It also "clanks". That's my wood, the temp and humidity that day, my other conditions, and all sorts of other variables, but it gives me a "good enough" indication as a backup to the other test criteria.