In Tantra, it's the bit about breaking conventional moral rules to ... something, something about using the power of forbidden things to achieve conventionally meritorious things.
What my teacher told me is that in tantra something that is karmically negative is not to be done ever. What can, and should be done, is taking something negative and turning it into something positive.
The analogy I use is; nobody wants to be bitten by a rattlesnake. But rattlesnake anti-venom is made from venom. If correctly transmuted it becomes medicine. But untransmuted venom is still poison. In Dharma, in order to take something negative/poisonous and correctly transmute it into something positive, you've got to have an enormous amount of positive aspiration such as refuge, renunciation, bodhicitta, at least a minimal understanding of emptiness, plus the specific training in whichever meditation is involved. This extraordinary skill is required for the transmutation to be real.
My teacher used the example of making a rule for somebody that they should not stab people, defined and "cutting someone with a knife." If you are trying to reform a thug, that type of rule needs to be made. But what if that thug then straightens out his life, goes to school and becomes a doctor? Is he then prohibited from performing a surgery? No. His extraordinary skill makes surgery something helpful. The next thug that needs to be reformed might see the surgery and object, saying, "He's still stabbing people, he's 'cutting them with a knife!'" That constraint is no longer applicable because of the greater skillfulness the doctor has acquired. But that is different than giving a doctor license to go out and revert to stabbing people, which is still prohibited.
In today's society this idea is widely abused. I personally believe that most of what is passed off as "crazy wisdom" and "skillful means" as such is simply a cover for bad conduct. But that isn't real Dharma. The way Vajrayana is taught is as if it is going to be properly and expertly practiced. That can and does happen, but it is not a given by any means. So, in my opinion, chances are that what appears to be vile behavior is simply vile--but sometimes