In practice
When price closes through a fair value gap instead of respecting it, the traders who relied on the gap are caught on the wrong side. The area then tends to work the other way: a failed bullish gap becomes resistance.
Traders use the flipped gap as an entry zone after a reversal, often as part of the sequence that follows a liquidity sweep.
What to watch for
A wick through the gap does not count; the flip requires a close beyond it. As with any flipped level, the first return is the most reliable and later ones matter less.