Some slot machines don’t just spin, reels stop, pays get paid and the spin is over. There’s a small subset of slots that can continue on when there’s a win, and one of the more common is usually known as Cascading, or Tumbling, Reels.
On these games, when a pay is awarded, the symbols that were paid then disappear. Any symbols above them drop down, new symbols are added to fill the spots that are empty, and if there’s fresh wins, the process repeats.
There’s a variety of games with this format, from Davinci Diamonds to Invaders from Planet Moolah to the Win it Again series of games. The format of the symbols dropping down allows for a variety of gameplay possibilities, and the manufacturers who use this format each have their preferred way to build up the wins.
Some use the format to trigger a bonus or extra payout after a certain number of cascades; others have bonus symbols that can drop in and if you get the requisite number of bonus symbols, the bonus will be triggered.
What tends to be fun about these games is that not only do you have the chances for multiple payouts from a single spin as the symbols keep disappearing and tumbling down, many games with this format have a free spins bonus that leverages the technique too, making the bonuses equally fun and sometimes lengthy, between retriggers and multiple wins per spin.
That is likely part of the reason why many games with this sort of format have been reissued and had sequels, keeping them in casinos in various forms to this day – the format remains popular among slot players accordingly.