How does the authors decision to open up the “Bull Whip” scene contribute to the overall meaning of the story?

The scene with the bull-whip is important as it tells us a lot about Delia's abusive relationship with Sykes. It also gives us an insight into Delia's tenacity as a woman, and how she's not prepared to let anyone drive her out of her own home.
Sykes wants Delia out of the way so he can install his mistress Bertha in her place. No amount of bullying or cajoling has succeeded so far, so Sykes resorts to outright trickery to be rid of his wife. He knows that Delia's absolutely terrified of snakes, so he lays a bull-whip across her shoulders to make her think there's a snake crawling across her back. Delia almost jumps out of her skin; but she won't back down. She paid for the marital home with the sweat of her labor and she'll be damned if anyone's going to drive her out.
Snakes are traditionally a symbol of evil, and Sykes later brings a real live one—a six-foot rattlesnake, no less—into the home to try and scare off Delia. But his wicked plan backfires spectacularly when the highly venomous creature bites and kills him. So once could see the bull-whip scene as foreshadowing this event.