How did Polyphemus plan to trap the men?

Polyphemus lived in a cave that used a giant stone slab as a door. This stone slab was so large and heavy that not even Odysseus plus his twelve men would have a chance at moving it. When Polyphemus let his sheep out, he checked to make sure Odysseus and his men were not hiding among the sheep (since his eye was gouged out, he had to rely on his sense of touch). However, since Odysseus and his men were hiding under the sheep, Polyphemus could not feel them out. So, basically the stone slab was Polyphemus' only plan to keep Odysseus and his men in.