Please look at the wording of 17(b)(1), emphasis mine:

Except as provided under paragraph © of this section, a financial institution holding a consumer’s account shall not assess a fee or charge on a consumer’s account for paying an ATM or one-time debit card transaction pursuant to the institution’s overdraft service, unless the institution:

The opt-in requirement is ONLY for ATM and one-time debit card transactions. For a preauthorized or recurring transaction like dinckelman was discussing, that does not apply.

Also some relevant sections of the official interpretation of 17(b):

2. No affirmative consent. A financial institution may pay overdrafts for ATM and one-time debit card transactions even if a consumer has not affirmatively consented or opted in to the institution’s overdraft service. If the institution pays such an overdraft without the consumer’s affirmative consent, however, it may not impose a fee or charge for doing so. These provisions do not limit the institution’s ability to debit the consumer’s account for the amount overdrawn if the institution is permitted to do so under applicable law.

8. Outstanding Negative Balance. If a fee or charge is based on the amount of the outstanding negative balance, an institution is prohibited from assessing any such fee if the negative balance is solely attributable to an ATM or one-time debit card transaction, unless the consumer has opted into the institution’s overdraft service for ATM or one-time debit card transactions. However, the rule does not prohibit an institution from assessing such a fee if the negative balance is attributable in whole or in part to a check, ACH, or other type of transaction not subject to the prohibition on assessing overdraft fees in § 1005.17(b)(1).

In summary:

• If it’s an ATM or one-time debit card transaction, you have to opt-in for them to charge you a fee
• If you didn’t opt-in, they are still allowed to cover the overdraft if they want to by taking from other accounts you have with them, they just can’t charge you an extra fee for doing so.
• If the overdraft is due to a preauthorized/recurring debit card transaction, a check, ACH, or other transaction not covered by 17(b)(1), your consent doesn’t matter, they get to charge you a fee.