The mad fientist explains it well.
The way I understand it is:
1. Put money in Traditional IRA or Traditional 401K.
2. Retire and have no income.
3. Roll 401K to tIRA.
4. Convert tIRA to Roth IRA
5. Take converted money out of Roth IRA after 5 years
A single person has a standard deduction of $6,200 and an exemption of $3,950 so they can have $10,150 of income and have $0 in taxes. The next $9,075 is taxed at 10%. If you have no income, you can convert $10,000 of a tIRA to a Roth IRA every year and not pay any taxes. Double that for a married couple. You can take conversions out of a Roth after 5 years at no penalty and no age restrictions.
So if you retire and live off taxable investments for 5 years while slowly coverting money in a tIRA to a Roth IRA, you will be able to take $10,000 out of your Roth IRA every year that had never been taxed. Double that amount for married couples.
A least that's how I understand it.