I've seen David Cay Johnston on a number of news shows over the years as a tax expert (he's good). He's spending a lot of his time on Trump now. In March he wrote, The real Trump tax scandal in USA Today.
It's all about tax rules that require you to depreciate, or reduce, the value of buildings over time, even if the market value of the structures is going up. If your depreciation is greater than your traditional income from work and businesses, Congress lets you report negative income. If these paper losses are just a dollar more than traditional income, it wipes out your income taxes for the year.
If Trump's returns show he has paid no income taxes in some years, that could be a reason he has not yet released details.
Congress says most Americans can deduct no more than $25,000 of real estate depreciation against their income. But if you work two days a week managing real estate and own enough that the depreciation exceeds your salary and other income, Congress lets you live income-tax-free. And for as long as you keep buying buildings and depreciating them, the tax does not come due."
There's debate now about what Trump might be hiding in his tax returns. Is it that he pays little or no tax? He says he works hard to pay as little as possible and people seem to support that. If using schemes like the above he manages to pay no tax, I don't think that will go over well with a lot of voters. The other theory is that they reveal that he's worth a lot less than he says. I wondered why a tax return (which reports yearly income not overall wealth) would reveal that, but the above scheme would fit with that evaluation. If he has to report depreciation on his current assets to avoid tax, returns for several years would show the value of those assets declining.