Sometimes the best investment advice comes from someone physically grabbing your arm to make sure you're listening. That's exactly what happened when Tony Robbins cornered Warren Buffett about wealth management strategy.

The encounter happened back in 2010, when both Robbins and Buffett were guests on NBC's "Today" show for an economy discussion. Robbins saw his chance for a private conversation before airtime. He'd long admired Buffett's journey from Nebraska stockbroker to investing legend, and he wanted to know how regular investors could navigate volatile markets.

Buffett's first response? A polite no. "He looked up at me with a twinkle in his eye," Robbins recounted in his book "Money Master the Game." The billionaire explained he'd already said everything there was to say on the subject.

But Robbins didn't give up that easily. He shifted tactics and asked what portfolio Buffett would recommend for his own family's wealth, especially now that he'd pledged most of his fortune to charity.

That's when things got physical. "He smiled again and grabbed my arm," Robbins wrote. "It's so simple," Buffett told him. "Indexing is the way to go. Invest in great American businesses without paying all the fees of mutual fund managers, and hang on to those companies, and you will win over the long term!"

The charity pledge Buffett mentioned dates back to 2006, when he committed to giving away the bulk of his wealth.

Robbins wasn't ready to accept defeat on the interview front. He enlisted some heavy hitters to help make his case. Hedge fund billionaire Ray Dalio and publisher Steve Forbes both reached out to Buffett on Robbins' behalf. Buffett declined again, but reinforced the same message: buy index funds for exposure to top companies and stay invested for the long haul.

Putting Money Where His Mouth Is

This wasn't just casual advice Buffett tosses around at television studios. He's been preaching the index fund gospel in shareholder letters, interviews, and public statements for decades. In December 2017, he even won a bet he'd made ten years earlier with Protégé Partners that hedge funds couldn't outperform index funds.

The most revealing insight came in Buffett's 2013 letter to Berkshire Hathaway shareholders. He laid out the exact instructions he gave his trustee for managing his family's money after he's gone.

"My money, I should add, is where my mouth is," Buffett wrote. "My advice to the trustee could not be more simple: Put 10% of the cash in short-term government bonds and 90% in a very low-cost S&P 500 index fund. I believe the trust's long-term results from this policy will be superior to those attained by most investors – whether pension funds, institutions or individuals – who employ high-fee managers."

There it is. The portfolio allocation that one of history's greatest investors chose to protect his own family's wealth. Not complex derivatives, not a sophisticated multi-manager approach, not even individual stock picks. Just 90% in an S&P 500 index fund, 10% in government bonds, and the discipline to leave it alone.