The Help page's definition says:
Brilliant Moves are always the best or nearly best move in the position, but they are also special in some way.
We replaced the old Brilliant algorithm with a simpler definition: a Brilliant move is when you find a good piece sacrifice.
There are additional conditions:
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You should not be in a bad position after a Brilliant move
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You should not be completely winning even if you hadn't found the move.
We are also more generous in defining a piece sacrifice for newer players compared to those who are higher-rated
A nearly best move would make the evaluation go down (since the evaluation is based on you playing the best moves).
Your move also threatens a royal fork on d6.
So I was really happy to finally beat the SonicFox bot (I'm below 1000 elo) and I was pleasantly surprised when I got my first brilliant move ever. However, when trying to understand why it was brilliant, I realized it was probably a blunder and should be marked with (??) rather than (!!). The move in question is this one:
So basically I'm threatening to go to C7 with my knight, forking the king and the rook, ignoring the threat on my other knight. The problem is, though, my 900 elo brain missed the fact that C7 is already protected by the knight on A6, and even then black could have just moved the rook I guess.
If that wasn't enough, the eval bar dropped from +0.8 to +0.5 after this move. I remember reading that brilliant and great moves on chess.com should never leave you in a worse position...so...is there actually any brilliancy in that move?