What is an inverse fair value gap (iFVG)?
A fair value gap is supposed to push price away. When a candle closes straight through it instead, the traders who sold into a bearish gap are now wrong, and many ICT traders expect the same zone to hold from the other side on the way back.
So a bearish gap that is closed through upward is read as a bullish inverse gap, and the trade is a long on the return into it. The same logic runs in reverse for a bullish gap closed through downward.