The first is that the manufacturer will put the Snell sticker in each helmet. The other is that Snell will random-sample the manufacturers' helmets from the marketplace, which is what really makes Snell unique when compared to other standards. This way, Snell has a specific, concrete way of assuring that the excellence evidenced during the certification process is ongoing throughout the production cycle. Snell buys helmets from distributors or stores for random testing. We prefer to buy from distributors because it's cheaper for the manufacturer.
Every manufacturer can slip once in awhile.
When a helmet does fail a random test, it may not fail by much. But a failure is a failure. We immediately notify the manufacturer, send them the test results and, if they wish, the helmet. Then, we go out and buy three more of that company's helmets. We test those in the exact places we tested the original random-tested helmet.