Here is how The New York Times editorial page describes it:
What the new rule would mean in practice is that limits on the amount of co-payments and the number of doctor visits or hospital days cannot be less generous than those that apply to most medical and surgical benefits. The same would be true of other rules, like those requiring prior authorization.
Here’s how Timothy Jost says it really works:
A financial requirement or quantitative treatment limitation is considered to apply to “substantially all” medical/surgical benefits if it applies to at least two-thirds of the medical/surgical benefits in a classification. A type of financial requirement or quantitative treatment limitation is considered “predominant” if it applies to more than one half of the medical/surgical benefits in a classification that are subject to the requirement or limitation. For example, if two thirds of the medical surgical benefits in a classification were subject to a coinsurance obligation, and more than half were subject to a coinsurance obligation of 30 percent, mental health benefits could be subject to a coinsurance obligation, but it could not exceed 30 percent.
Where different financial requirements or treatment limitations apply within a classification, plans may combine levels until the combination of levels applies to more than one half of the medical/surgical benefits subject to the requirement or limitation, and apply this requirement or limitation to mental health benefits. The portion of medical/surgical benefits subject to a financial requirement or quantitative treatment limitation is based on dollar amounts for plan payments for the benefits expected to be paid under the plan, before enrollee cost-sharing requirements are applied. So, for example, if 20 percent of payments for medical/surgical services were for services in a classification with a 10 percent coinsurance level, 40 percent for services with a 15 percent coinsurance level, and 40 percent for services with a 20 percent coinsurance level, the plan or issuer could apply a 15 percent coinsurance level to mental health and substance abuse disorder services. If plans offer coverage through separate insurers for mental health or substance use disorders on the one hand and medical/surgical services on the other, parity calculations must apply to combined coverage as a whole.