No no the key is you DON"T roll over to a Roth while working, you do it after you're retired in a lower income bracket. I'll use real numbers so it makes sense. Here are some approx. tax brackets for a single person (off the top of my head):
$0-8,000 10%
$8001 - $16000 15%
$16001-$36,000 15%
$36,001-$86,000 25%
Note: These brackets are for taxable income, which is your income less your deductions. Everyone gets $5,800 and $3,700 in deductions automatically (standard deduction and exemption, respectively). So really the 15% bracket doesn't kick in until $16,000 of income.
You put $17,500 per year into your 401k while working, you're in the 25% tax bracket, so you save $4375 per year in taxes.
You work for 10 years, so right before retirement your situation looks like this:
401k balance $175,000 (17,500 x 10 years)
Cash balance $43,750 (4,370 x 10 years)
You quit work in year 11, and roll $8,000 into a roth ira. You live off of $8,000 cash this year. The standard deduction and exemption bring your taxable income below zero. You pay no taxes on the Roth IRA conversion because your taxable income is so low. The next 4 years are exactly the same, and at the end of 5 years you have:
401k balance $143,000
Roth IRA balance $32,000
Cash balance $11,750
You can now pull out $8,000 out of the Roth IRA tax and penalty free this year (the 5 year anniversary of your first rollover), you can also roll $8,000 more out of the 401k tax free this year. Repeat until the 401k is completely transferred to Roth. You got $43,750 in free money.