"We had to be creative to try to find a way to make the responsibility for acting in your client's best interest—the fiduciary responsibility—enforceable in the IRA context," explained Borzi.

The result of that creativity was the rule's Best Interest Contract Exemption provision.

Under the BIC exemption, advisors will be contractually bound to act as fiduciaries when recommending a rollover to an IRA, and also when recommending investments within the accounts.

Borzi acknowledged what critics of DOL argued throughout the regulatory process—the DOL does not have direct enforcement authority over IRAs.

That means the BIC exemption will not be enforceable relative to IRAs through statutory rules, beyond the existing excise tax on prohibited transactions, which Borzi said has proven to be a "big deterrent" in recent enforcement actions.

Absent the statutory authority, the BIC exemption will have to be enforced through private legal action, or as Borzi put it, "the consumer has to enforce the rules through state contract actions."

In interpreting the rule's application, Borzi advised retirement advice providers to err on the side of caution.

The rule was crafted broadly by design, she said. "When in doubt, assume that you are supposed to be under the BIC exemption," added Borzi.

Nor should advisors expect sweeping exemptions to the rule once it is implemented next year. "Any exemption to the rule will be construed extraordinarily narrowly," she said.

Since the rule's finalization in April, Borzi and her team have already been active fielding questions from industry, some of which have derived from creative interpretations of the rule, she said.

Borzi pledged to keep her door open going forward, and said the agency will use its ability to issue additional guidance through fact sheets and other agency interpretations, "as a way to be more nimble if we see problems and practices that don't fit the spirit of the rule, if not the letter of the rule."

She likened that likely additional guidance to the many fact sheets issued since the passage of the Patient Protection and Affordable Care Act, the enforcement of which Borzi and EBSA also oversee.

In the end, the extent of enforcement of DOL's fiduciary rule will rely on investors, and attorneys, to determine when advisors are in breach of the BIC exemption.

"We can't be everywhere," said Borzi. "We will monitor what is going on in the market place aggressively."

Borzi also called on fiduciary advocates to assist the Department in understanding how advisors will operate under the rule.

"There were not a lot of choices to deal with enforcement," she added. "We are going to have to monitor this carefully."