Something called rolldown. Something about large pots that don’t get won then rolldown to lower winning tickets in future drawings?
A little more in the Wiki page on the movie and this:
Cash Winfall
Cash Winfall was drawn Mondays and Thursdays. 6 numbers 1 through 46 were chosen. The jackpot began at $500,000, it always was paid in lump sum. Lower-tier prizes were $4,000, $150, or $5 for matching 5, 4, or 3 numbers respectively, 2 numbers won a Cash Winfall bet. If the jackpot reached $2 million and was not won, the jackpot was “rolled down” with the secondary prizes increased.
After the Boston Globe published reports of individual stores selling millions of dollars in tickets, state officials suspended the game, suspecting organized crime involvement. Investigations revealed that the profiteers were a retired couple from Michigan (who had exploited the same system when that state’s lottery had the Winfall game) and a group of MIT college students who, by legally exploiting elements of the game, were practically guaranteed to win profits of approximately 20% when tickets were bought in rolldown conditions.[4] The story was adapted into the 2022 Paramount+ film Jerry & Marge Go Large.
Cash Winfall ended on January 26, 2012.[5]