There is some debate about this (to say the least) but the theory is that "hot sections" of the deck are propagated between shuffles of the decks when hand-shuffled. So if you run into a streak of good cards at the beginning of one shoe of cards, then that means you'll have another streak of good cards mid-way through the next shoe (or whatever). Then you can plan your bets accordingly by placing small bets to minimize your losses during the cold sections and then upping your bets during the period of hot cards.