It involves taking money out of a U.S. tax advantaged financial instrument for its stated purpose then contributing a portion of it back again when not all of the distribution was needed / used.
Earnings and principal were distributed to the owner but when it's contributed back again it all gets recharacterized as basis (which isn't subject to tax or penalties if not used for its specific purpose). Custodians asked for guidance in situations where this occurred and that's the guidance the IRS gave them.
I don't think you could make massive sums of money with it, but Drinky Crow could probably buy a new Aprilia.
If you want to know specifics we can talk in camera.