Early in the story, Hurston writes:
Just then something long, round, limp and black fell upon her shoulders and slithered to the floor beside her. A great terror took hold of her. It softened her knees and dried her mouth so that it was a full minute before she could cry out or move. Then she saw that it was the big bull whip her husband liked to carry when he drove.
This shows from the start that Sykes will use the weapons he has at hand to try to control Delia. A snake or a whip are one and the same to him, because they are both agents of terror and mastery.
Sykes oppresses Delia to feel better about himself as an unemployed Black man unable to get ahead in the racist South of his time period. He has little control over his life, so he torments Delia because he can.
However, the bull whip scene is significant because in it, Delia fights back against him for the first time. Scaring her with the whip and then scattering and dirtying her laundry become the final straws after fifteen years of putting up with his abuse:
Delia's habitual meekness seemed to slip from...
her shoulders like a blown scarf. She was on her feet; her poor little body, her bare knuckly hands bravely defying the strapping hulk before her.
Sykes insults and abuses her work doing laundry for white people seven days a week because it reminds him of their humiliating situation in a racist society. Yet Delia, fighting back, tells him that it is her "sweat," a life of almost endless toil, that keeps him fed and housed. She defends herself from him with an "iron skillet," and he is intimidated, so he does not hit her as he usually would.
Delia's newfound voice and courage after the encounter with the bullwhip foreshadow the same agency she will show when she doesn't save Sykes from the snake bite at the story's end.