Automatic flush allows you to transfer all remaining funds from a fixed card back to the main pool of funds after it's been used for a payment. The flush occurs once the payment is settled.
Case Study
You set up a new application for Slack, which charges once a month based on usage during the billing period.
The subscription base fee is £250, but since the total invoice amount varies month to month, you set a budget of £400 for the application and enabled Automatic flush.
Say your low usage this month means that Slack is charging you £250 only. Once your card is charged and the transaction is settled, the remaining £150 in the card will be automatically sent back to your Available balance so that other apps can make use of those funds.
No monitoring or action is required.
This is a great functionality for apps and services whose subscription charge varies, as it allows you to free up your capital once the payment has gone through and avoids hoarding unused funds among multiple cards.
When not to use Automatic flush
If you expect to make more than one payment per cycle, you should not use Automatic flush because once the first payment is taken from the card, the remaining funds will be flushed, and thus other payments within the same billing cycle won't be processed automatically.