You are correct if you are referring to what most people call a Roth conversion. This alternative is different. Essentially, you contribute to an IRA on a post tax basis. Then (typically immediately) you convert that post tax IRA to a Roth. It is tax free so long as you have no other IRAs. It creates a "back door" to a Roth IRA that isn't allowed by rules to fund directly.

If you have another IRA (versus only a 401K), there are proration rules where you prorate between all of your IRAs, what is taxable (the other IRAs) and what is not taxable (the post tax contribution you made this year). The worst part isn't the taxable income from the prorated piece of your other IRAs, the worst part is now you have a residual post tax IRA that you and your heirs have to keep track of that tiny proportion of the $7K or $8K you contributed post tax to an IRA for the rest of your life and into their life (or multiple children prorated into smaller amounts to keep track of and prorate as the retirement account is drained) until it is cashed out.

That is what I call a recordkeeping nightmare...or you just end up paying taxes on those after tax contributions (plus growth for a lifetime) which defeats the purpose of doing it in the first place.

Back to the original question, the reason you would move any IRA back to your 401K is so that you could do a back door roth each year. I would never roll back my IRA's into a 401K. To me, that isn't a good enough reason to do it.