Cascading reels — also known as collapsing, tumbling or avalanche reels — change what happens after a win. Instead of the spin ending, the symbols that formed the winning combination disappear, the remaining symbols fall into the gaps, and new ones drop in from above. If the new layout creates another win, the process repeats.
The chain continues until a cascade produces no new win, all within the same paid spin, much like a free respin earned by success. Many games add a multiplier that rises with each consecutive cascade, so the later wins in a chain can be worth more than the first.
The mechanic is common in cluster-pays and ways-to-win slots and inside free spins rounds. For operators, it makes a useful lobby tag for players who enjoy chain reactions, and providers list it among a title’s core features.