The constant proportion strategy in roulette is very similar to the constant bet strategy, but instead of placing bets in constant amounts, the value of your bet will correspond to a certain percentage of your bankroll.
For instance, a player has a bankroll of €100 and wants to bet 10% on a certain colour on each round of the game. On the first round he makes a bet of €10 (10% of €100) and wins. This leaves your bankroll with a value of €110, so on the next round he would bet €11 (10% of €110). And keep betting in this way until he wins a satisfactory amount, loses everything or simply chooses to stop playing.
As already mentioned, this strategy is very similar to the constant bet strategy, but with an added element. If you keep winning, your bets will increase, which gives you more chances to win big. On the other hand, if you keep losing, your bets will decrease, which means you will lose at a slower rate. This self-regulating mechanism makes the constant proportion strategy a little more interesting than the constant bet one.