The payment is triggered automatically after the booking has been completed. The payment process for SEPA direct debit can take 6-10 working days until the amount is finally debited. This process cannot be controlled manually.
If your bank account does not have sufficient funds at the time of debiting, your payment will fail and your user account will be restricted for the time being until you have settled the outstanding payments directly via credit card, GiroPay or SOFORT, including the applicable fees.