Suppose you're at a full table and 12 of the last 20 cards are aces or tens (rare but possible). These get shuffled and cut into the first three decks, giving you a true count of four (12/3) for the first half of the shoe, which is significant.
We never really put it into practice, though, since: 1) you have to track the count for the last 20 cards in addition to the regular count, 2) shuffles change, 3) dealers are inconsistent, 3) casinos use different shuffles, 4) the typical advantage is likely to be much smaller.
My knowledge on this is at least 20 years out of date, though, so who knows?