This is for paying less tax when converting to Roth

You need to pay tax on top of your current tax for converting deductible IRA to Roth IRA. The IRA which rolled over from 401k is deductible IRA. If you would like to convert all Rollover IRA to Roth IRA, then you should not roll over back to 401k.

If you have a separate nondeductible IRA, you just want to convert this nondeductible IRA without any earnings to Roth IRA, you don't need to pay any tax for conversion if you roll over back your Rollover IRA to 401k.

If you don't roll over back it to 401k and there are lots of money in it, you need to pay substantial tax even you just convert the separate nondeductible IRA to Roth IRA. There is a formula you can find in IRS website.

IRS stated that you can't convert only nondeductible portion of IRA to Roth IRA.

Another note, no one else is allowed to touch your 401k, such as bankrupcy or legal issues.