Most people know their 401(k) has a contribution limit. The number that changes every year, the one you try to max out.
That is not the real limit.
There is a second, much larger ceiling that covers everything going into the plan. Your deferrals, your employer's match, and a third bucket almost nobody uses.
The gap between those two numbers is enormous. For many people it is more than double their personal limit, sitting there unused every single year.
A minority of plans let you fill that gap with after-tax contributions and then move the money into a Roth. When it works, it is the single largest Roth opportunity available to an ordinary employee, several times bigger than the regular backdoor Roth.
When it does not work, it is because your plan lacks one specific feature, and finding out takes one phone call.
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Here is the part that should annoy you. Most people who have access to this never learn it exists, because it is buried in a plan document nobody reads and no benefits presentation has ever mentioned it.