Why a paycheck with nothing in it is matched with nothing
Most plans work out the employer match one pay period at a time, on whatever you contributed in that period. It is a sensible way to run payroll and it has one consequence nobody mentions: a pay period where you contribute nothing is matched with nothing, and the match for that period is not carried forward or made up later. It simply does not happen.
That only matters if you have periods with no contribution, and the commonest reason for having them is reaching the annual deferral limit before the year ends. Contribute at a rate that exhausts the limit in August and the September, October, November and December paychecks have nothing in them to match.
What the provision does
A true-up looks at the whole year once it has finished and asks a different question: given everything this person contributed over twelve months, what should the match have been? It then pays the difference. Plans that have one usually settle it in the first quarter of the following year, which is why people who have one often do not realise it. The money arrives months later and is easy to mistake for something else.
The provision is not exotic and it is not rare. It is also not universal, and there is no way to tell from the outside which kind of plan you are in.
How to tell whether yours has one
The summary plan description is where it is written, usually in the section on employer contributions. The wording varies, so it is worth searching the document for “true-up” and, if that finds nothing, for “annual” near “matching contributions”.
If the document is ambiguous, the question to put to your plan administrator is narrow enough to get a straight answer: does the plan true up matching contributions after the end of the plan year? That is better than asking whether you can “max out early”, which invites a yes about the contribution rather than about the match.
Why the answer changes what you should do
With a true-up, the pace of your contributions is close to irrelevant to the match, and front-loading costs you nothing beyond the timing of when the employer money lands. Without one, pacing is the whole game, and the difference is real money rather than a rounding detail. Our 401(k) true-up calculator puts a figure on it for your own salary and match formula, and front-loading 401(k) contributions covers the cases where doing it anyway still makes sense.