1. after tax 401K is one way of 提前交税.
Other ways of 提前交税 includes Roth IRA and Roth 401K.
2. If you have mopney, you should max out in the following order:
Roth IRA, Roth 401K and After tax 401K.
3. If you want to rollover 401K to 提前交税, you can only rollover to Roth IRA.