Paroli is a positive progression betting system, usually applied to even-money wagers such as red or black in roulette. The player picks a base unit and raises the stake only after a win, aiming to make the most of short winning streaks while risking little during losing runs. It is the mirror image of the Martingale, which raises stakes after losses.
The cycle is simple: bet one unit; after a win, double the stake; after a loss, go back to one unit. After three wins in a row, bank the profit and start again. With a $5 unit, a full cycle runs $5, $10 and $20 and ends $35 ahead. Like every betting system, Paroli changes the pattern of results, not the house edge.
For operators, recognisable progressions like this show up in betting data and are worth keeping in mind when writing bonus wagering rules and responsible gambling messages.