The 401k-Roth pipeline is probably best illustrated with a hand-wavy example. Here's one that drastically oversimplifies things (by ignoring gains from investments and using totally made up tax numbers, etc) but should get the point across.
My COL for a year is $25K. I have $800K in my 401k. I have $25K in my Roth IRA. I have $125K in my taxable accounts, which I can take out with no penalty.
I retire.
YEAR ONE!
I pay for my COL with 25K from my taxable account. I roll $30K from my 401k into my Roth IRA, paying $5K in income taxes on it.
At the end of the year, I have $50K in my Roth IRA, $25K is "one year old." I have $100K in my taxable accounts, which I can take out with no penalty.
YEAR TWO!
I pay for my COL with 25K from my taxable account. I roll $30K from my 401k into my Roth IRA, paying $5K in income taxes on it.
At the end of the year, I have $75K in my Roth IRA, $25K is "one year old", $25K is "two years old." I have $75K in my taxable accounts, which I can take out with no penalty.
YEAR THREE!
I pay for my COL with 25K from my taxable account. I roll $30K from my 401k into my Roth IRA, paying $5K in income taxes on it.
At the end of the year, I have $100K in my Roth IRA, $25K is "one year old", $25K is "two years old," $25K is "three years old." I have $50K in my taxable accounts, which I can take out with no penalty.
YEAR FOUR!
I pay for my COL with 25K from my taxable account. I roll $30K from my 401k into my Roth IRA, paying $5K in income taxes on it.
At the end of the year, I have $125K in my Roth IRA, $25K is "one year old", $25K is "two years old," $25K is "three years old," $25K is "four years old." I have $25K in my taxable accounts, which I can take out with no penalty.
YEAR FIVE!
I pay for my COL with 25K from my taxable account. I roll $30K from my 401k into my Roth IRA, paying $5K in income taxes on it.
At the end of the year, I have $150K in my Roth IRA, $25K is "one year old", $25K is "two years old," $25K is "three years old," $25K is "four years old," and $25K is finally five years old. I have $0K in my taxable accounts.
At this point, I can take out $25K from the Roth with no penalties of any kind, and I have a consistent pipeline of money coming out yearly from my 401k. I can increase this amount as necessary to cover changes to COL, etc. This whole example ignores gains, compound interest, etc. and isn't meant to do anything but explain the pipeline in case the previous example didn't make sense.
Hope this helps.