What is a Fuzzy Guard?
Fuzzy Guard (aka Fuzzy Block) is very similar to Guard Switch, but not using it against any specific setup. You are basically hedging your bets and preemptively inputting ~~ to block high for a brief moment to block an opponent's overhead attack before going back to blocking low.
Since you are switching to standing block, you will be vulnerable to lows during that time. Blockstrings that go into a low like 2A > 2B, or 2P > 2K will hit you should you attempt to fuzzy guard as soon as you block the first hit.
While this can be slightly mitigated by only guarding high for a very short amount of time, it also means that your window for blocking overheads becomes smaller. Thus the best way to use this is to intelligently pick a spot when you think the opponent might got for an overhead and Fuzzy Guard only during that time (almost like a Guard Switch).
Recognizing When the Opponent is Using Fuzzy Guard
When the opponent uses fuzzy guard, you will see them switch from blocking low to blocking high, then back to low again. If you see them fidgeting between low and high block when you're not doing anything, they're probably Fuzzy Guarding.
You can also recognize this in the middle of your blockstring by paying attention to the opponent - did they switch between crouching and standing as they blocked a few hits in your blockstring? That probably means they're using Fuzzy Guard (or Fuzzy Jump). Next time go for a low!