Price gaps up, trades for one or more sessions in a narrow area, then gaps back down below where it started, leaving the cluster floating with untraded space on each side. The bottom version is the mirror.
The reading is that a group of participants bought in a price zone that the market then abandoned entirely, leaving them trapped. Islands are rare, and the ones that form after an extended trend with an exhaustion-gap on the way in are the most cited.
Rarity is also the problem: there are too few examples to make confident claims about hit rates, and the pattern is confined mostly to markets with real overnight gaps, which means daily equity charts rather than round-the-clock futures or crypto.
Related: exhaustion-gap, abandoned-baby, reversal-pattern, gap, kicker-pattern