Have you ever made a brilliant move that you didn’t know why it was brilliant?


Yes, they changed the definition several years ago. They decided that trying to define a brilliancy was too difficult.
They decided to define any decent sacrifice as a brilliancy and to allow lower standards for lower rated players.

From the definition in the Support pages:
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.