Trivia: Writer Richard Matheson based this story on a true incident. His young daughter fell out of bed one night and rolled against the wall just far enough away that when he reached under the bed, he at first couldn't find her. This inspired both a short story and this TZ script, where a child falls into another dimension.
Trivia: The inspiration for The Simpsons Treehouse of Horror VI segment "Homer3."
Answer: It's explained in the episode. The aliens are experimenting to see how humans react to various scenarios. This just happens to involve them using super strength.
Thanks. It's been a long time since I saw the episode so I forgot.