The provision is spelled out in the Pay With My Refund Agreement, which you agreed to when you opted to pay with your refund.
In addition, by clicking the "I Agree" button below, you electronically sign and authorize Intuit (through its third party processor) to debit the TurboTax Fees, Additional Products and Services Purchased and any applicable sales tax from your bank account number identified in the Agreement below, in the event that you do not receive a tax refund that is sufficient to pay for them.