EXCLUSIVE: This is possibly the most often discussed, and misunderstood, topic among money types from Wall Street to Hollywood. For good reason. Through his 80% ownership interest in theater chain National Amusements, and the magic of super voting shares, Sumner Redstone controls close to 80% of the votes at Viacom and CBS – companies in every dealmaker’s sights. He turns 92 in May. And investors imagine lots of potential scenarios about what might happen when he goes: While bits and pieces of the plans have come out over the years, there’s no public document that spells out in detail who will control his shares when he leaves the stage, or how the potentially hefty estate taxes likely would be handled.

Here’s the answer, as told to me by people in position to know. It gets technical, but it’s important information for anyone who wants to understand what could happen to Viacom (which owns cable networks including MTV, VH1, Comedy Central, and BET, as well as Paramout) and CBS (owner of the No. 1 broadcast network, TV and radio stations, Showtime, and book publisher Simon & Schuester).

Redstone’s National Amusements interests will be controlled by a five-person trust. But it would manage the shares on behalf of his five grandchildren and future generations — not his two children, Shari and Brent. The irrevocable trust is much like one used by the E.W. Scripps family, and would dissolve on the passing of the last heir who’s alive at the time of Redstone’s death.

All of the trustees are lawyers, and they’d be divided into two groups. Three are not members of the Redstone family, and therefore deemed to be independent trustees: They are Viacom CEO Philippe Dauman, David Andelman (a CBS director), and George Abrams (a Viacom director, and Redstone’s long-time attorney).

The two additional members belong to the family: Shari Redstone – who’s National Amusements’ President (she owns the 20% that Sumner doesn’t have) and Vice Chair of the Viacom and CBS boards — as well as one of the media mogul’s grandchildren.

There’s no designated chairman, and all decisions would be made by a majority vote. If an independent member leaves, then the other independent members would pick a replacement. Same thing among the family: The remaining family member would pick another trustee from among Redstone’s descendants.

As for Brent: Sumner bought out his son’s one-sixth stake in National Amusements in 2007 to settle Brent’s suit claiming that he had been unfairly shunted aside in the family business. But his children are included among the beneficiaries of the trust.

What about the estate taxes? Many estimate that the taxable gains from National Amusements’ investments in Viacom and CBS would be so huge that they would require Redstone or the trustees to sell something — with rumors including everything from Paramount to all of Viacom or CBS. The theater chain bought Viacom in 1987.

The bill would probably amount to about $1 billion — depending on the value of Viacom, CBS, and other National Amusements properties at the date of Redstone’s death. That could be paid off over a decade or so with minimal interest, assuming that today’s low interest rates hold. The thinking is that the trust could handle the payments with dividends and other distributions from Viacom and CBS. In a pinch, the trust also could sell some of the Viacom and CBS shares without giving up voting control of the family’s media empire.

Once Sumner goes, then Dauman likely would pick up the chairman title at Viacom, with CBS chief Les Moonves doing the same at his company. Their contracts say that they don’t have to report to someone other than Redstone.

Technically speaking, though, nothing else would change at the media powers. Each company’s board would remain intact, and the majority shareholder would still be National Amusements. The only difference is that control of National Amusements would shift from Sumner to his trust.

So, if someone wanted to buy Viacom or CBS — or make any kind of deal — the offer would go to the target company’s board. Directors would be legally obligated to represent the interests of all shareholders, not just National Amusements. As a practical matter, though, the board would have to seriously consider the Redstone trust’s wishes. It would probably hire an independent banker to help decide what outcomes would serve the family’s long term interests.

Why doesn’t Redstone just set everyone straight on his estate plans by releasing details himself? He doesn’t have to: Although it could indirectly have a profound affect on publicly traded Viacom and CBS, it only directly affects National Amusements, a private company.

And here’s something else to remember: He can always make changes. Or, as he has vowed, perhaps live forever.