A distribution paid to you (whether or not it is your intent to roll it over to an IRA) is subject to 20% minimum federal tax withholding (not 10%) on the portion of the distribution that would be taxable if not rolled over. As Opus 17 said, to complete the rollover of the entire distribution made this way would require you to substitute other funds for the portion withheld for taxes. You'll then be credited with the tax withholding on your tax return, increasing your refund or reducing your balance due, depending on your other income and tax payments.
Also as Opus 17 said, direct rollovers from the 401(k) are not subject to mandatory tax withholding. Even though the rollover from the traditional account in your 401(k) to a traditional IRA or from the designated Roth account in the 401(k) to a Roth IRA do not result in taxable income, they are still reportable on your tax return.
