In practice
A mitigation block forms when a trend fails without first taking out the previous extreme: price makes a lower high, for example, and then breaks down. The last up-move before the break becomes the zone.
When price returns to it, traders who bought there get a chance to exit near their entry, which adds selling. It is used as an entry zone in the direction of the new move.
What to watch for
Because there was no liquidity sweep first, it is considered weaker than a breaker block. Treat it as a secondary zone and ask for more confirmation before trading from it.