On paper, his $5.7 billion donation vaulted Musk up the ranks of the country’s most generous philanthropists in 2021—second only to the $15 billion donated by Bill Gates and Melinda French Gates to their joint foundation last year, according to the Chronicle of Philanthropy. But it’s also possible that Musk hasn’t yet given away a single cent of this largesse.

Instead, several philanthropy experts have speculated that Musk has merely moved his money into intermediary investment vehicles known as donor-advised funds, or DAFs. These funds hold assets that are earmarked for charity, allowing the donor to take immediate tax deductions against their income for the year the gift is made—but without any obligation or deadline to actually distribute it.

“Money can sit in donor-advised funds for a long, long time,” Elizabeth Dale, an associate professor of nonprofit leadership at Seattle University, tells Fortune. “We don’t have a lot of [regulatory] policies around these newer forms of giving.”

The tax benefits are one reason that donor-advised funds are an increasingly popular—and increasingly controversial—vehicle for wealthy tech entrepreneurs and other billionaires. They’ve been adopted by many of the country’s billionaire philanthropists (who otherwise have some very different styles of giving), including MacKenzie Scott, the writer and former spouse of Amazon founder Jeff Bezos; Bill Ackman, the activist investor and founder of Pershing Square Capital Management; Mark Zuckerberg, the founder and CEO of Facebook (now Meta); and Jack Dorsey, the founder of Twitter and Square (now Block). And they’re becoming more popular: In the 2020 fiscal year, contributions to DAFs grew by 21%, to $31.7 billion, the Chronicle reported late last month.

Their appeal goes beyond tax advantages: Donor-advised funds are relatively low-cost and easy to establish, especially when compared with the old-school route for the wealthiest donors of setting up a private foundation (and all the staffing and other overhead costs that such organizations can entail). Instead, donors park their assets in an account administered by a sponsoring financial institution, such as Fidelity or Vanguard, which holds on to (and invests) the money unless and until the donor directs them to start writing checks. These accounts are also far more accessible to a wider swath of would-be philanthropists. For example, Fidelity—the largest DAF sponsor—has no minimum for individuals opening a charitable account, and says that its annual fees average out to about 1% of the balance.

“You don’t need to be a billionaire or even a millionaire to open a DAF,” Dale points out. “They are much more accessible to ‘everyday’ donors than a private foundation would be.”

These accounts can hold cash and stock, as well as cryptocurrency, real estate, and other alternative investments. And they are, by some measures, more generous than traditional foundations: Grants from DAFs increased 27% to $22.4 billion in fiscal 2020, almost twice the amount distributed by the 10 largest private foundations during that time, according to the Chronicle.

DAF defenders say that their current structure allows billionaires to earmark money for charity and then thoughtfully study how best to deploy it. (It also means that if the underlying investments pay off, the ultimate value of the donations will grow.) But these funds have many critics, who argue that the accounts enable billionaires to take tax write-offs without actually giving away any money. Unlike private charitable foundations, which are required by law to pay out at least 5% of their assets on an annual basis, DAFs have no payout requirements or deadlines. (If donors die before their DAFs finish disbursing their money, and they haven’t left instructions for continued payouts, either their named successors take over or the DAF sponsor can distribute the money to charities.)

A bipartisan bill now under consideration by both houses of Congress would put more restrictions on DAFs, by creating new types of accounts; one would provide upfront tax benefits only if deposits are paid out within 15 years, while another would give donors 50 years to distribute funds but would not provide upfront income-tax deductions.

DAFs’ tax benefits for donors can also eclipse those of routing money to a private foundation. For example, Musk would be able to claim a DAF-related tax deduction of as much as 30% of his 2021 adjusted gross income, versus only a 20% deduction if he routed that money to his private foundation, the Associated Press reported last month. That sort of offset could take the sting out of a particularly big tax liability for Musk, who has sparred with Sen. Elizabeth Warren and others over how much he pays in taxes. (He sold more than $16 billion worth of Tesla shares late last year, and said in December that he was staring down a tax bill of more than $11 billion.)

Musk, the poster boy for what the New York Times in December dubbed “troll philanthropy,” last year also flirted with donating $6 billion to the United Nations World Food Programme. “If WFP can describe on this Twitter thread exactly how $6B will solve world hunger, I will sell Tesla stock right now and do it,” he tweeted in October. That post, followed by his disclosed $5.7 billion donation of Tesla shares to charity, initially raised speculation that Musk had followed through—but the World Food Programme has not yet received any donations from Musk, a WFP spokesperson told Fortune last week.

Perhaps the nonprofit’s initial reply to Musk—“$6B will not solve world hunger, but it WILL prevent geopolitical instability, mass migration and save 42 million people on the brink of starvation,” WFP executive director David Beasley argued on Twitter—didn’t pass the ultra-billionaire’s muster. But given Musk’s own levels of philanthropic disclosure this year, there’s a certain irony in another one of his conditions for donating to the WFP. The nonprofit must provide “open source accounting,” Musk wrote in October, “so the public sees precisely how the money is spent.”