At the end of each week, you have the option to roll over your remaining Safe-to-Spend into the next week. The rollover is cumulative which helps ensure you are spending less than you earn on average.
If your Safe-to-Spend is positive, rolling over the remainder will increase your spending power for the following week. Turning off the rollover will keep the extra money safely in your bank account.
If your Safe-to-Spend is negative we generally recommend rolling that amount forward and working to catch up on your spending the following week. Sometimes, however, it's better to get back on track than to try to keep catching up. If catching up feels like a non-starter then cut your losses, turn off the rollover, and start fresh.