The same level, broken twice: once with one aggressive candle that skips prices, once with a slow grind that trades every one of them. Only one of the two leaves anything behind.
What it actually is — Speed, not size. A candle that closes well beyond the level and barely overlaps its neighbours has skipped prices, and the gap it leaves is what proves it.
Why the gap matters — It is the only objective evidence that a break was aggressive. A break with no imbalance behind it has nothing to distinguish it from drift.
How to use it — Displacement tells you the break is real; the gap it leaves tells you where to act from. One without the other is half a signal.