Underwriting happens once, on the day you apply, and the health class you get that day is the one the carrier prices for as long as the policy lives. Any coverage you add later gets underwritten again, at whatever your health is then. This rider is the exception. It gives the owner the right to buy more coverage on set dates, with no new exam and no new health questions. The added coverage keeps your original health class.
The mechanics vary by carrier but the shape is consistent. There's a schedule of option dates, often every few years from the issue age up to somewhere in the early forties. There's a maximum amount per option, set at issue, and the amounts don't accumulate: skip one and it's gone. Many riders add life events as extra option dates. A marriage, a birth, an adoption, or a home purchase opens a window too. Usually a short one, measured in weeks. The new coverage is priced at the carrier's current rate for your attained age, in your original class. So it isn't cheap coverage. It's coverage that can't be declined.
The premium for the rider itself is small, priced per thousand of option amount, and it usually drops off when the last option date passes. On a policy bought at 25 that makes it one of the cheaper things on the contract.
Where it matters for a cash value design is that added coverage is added room. A policy funded to the limit in year one has nowhere to put more money. Add more and it turns into a MEC. A new layer of death benefit under the rider raises the ceiling on what the policy can take. That's how a policy started small at 24 grows with an income instead of being replaced at 34 by a second policy with fresh charges and fresh health questions. Whether a rider exercise resets the seven-pay test depends on the contract and the size of the increase, so ask before you count on the room.
The rider matters most on policies for children and young adults. A diagnosis in the twenties would otherwise end the ability to add coverage for good. With the rider, the right to keep buying survives the diagnosis.
If you're buying a policy for a child or a young adult, ask for the rider by name and for the largest option amount the carrier will issue. It's on the same short list as the riders that matter. If you already own a policy with it, the option dates are on the schedule page of the contract. Put every one of them on a calendar with a reminder sixty days ahead. The window closes on its own, and the carrier isn't obligated to remind you.