A post-tax contribution will grow tax-deferred but when it leaves the 401(k) the gains will be taxed as ordinary income.

When money leaves a 401(k), unless it's Roth or a recharacterized post-tax contribution, its taxed as ordinary income.

By recharacterizing it to Roth you'll be able to withdraw the gains on that money tax free. It's the same as making a Roth 401(k) contribution.

As an aside, if you put $10,000 pre-tax in your 401k and 20 years later you have $5,000, the withdrawals will still be taxed as ordinary income even though you have a "loss". So think of a 401(k) withdrawal as a tax-filter. It catches taxes on the way out. There are two ways to avoid that filter: Roth contribution or After-tax Roth recharacterization.

I'm not a tax professional but that is my understanding of the system as it exists now.