The breakout candle skips a whole slice of price on the way up — the gap it leaves behind is what price often returns to test.
Downtrend
Price is trending down in strong, uneven impulses — lower highs and lower lows.
TRENDRANGESWEEPSETUPGAP
Alternates bullish / bearish automatically
The fill doesn't always look like this
Filled completely — like the animation above: price trades all the way through the gap before reacting.
Only to the midpoint (CE) — many reactions happen at the gap's 50% level (consequent encroachment), well before it's fully filled.
Never revisited — in a strong enough move, price can leave the gap open for a long time, sometimes for the rest of the trend.
Already tapped intrabar — on a lower timeframe (1m and below), price can dip into the gap while the higher-timeframe candle is still forming. On the HTF the tap never shows.