As soon as you have activated/deactivated the function within your app, it will take effect. 

This means:
If you've activated the feature, all payments you make from then on will be rounded up. All payments that are reserved on your account will also be rounded up once they are actually debited.

If you've deactivated the feature, all payments you make from then on will no longer be rounded up. All payments that are reserved on your account will also no longer be rounded up.