He saw her from the bottom of the stairs
Before she saw him. […] (1-2)
These lines set up how the power dynamics in this relationship will shift throughout this piece in a physical sense. At this point in the poem, the woman is at the top of the staircase, and knows something her husband doesn't. So she's at the top of the food chain in this relationship right now.
Mounting until she cowered under him. (11)
Power shift alert. This part of the poem is where the man starts to take control. This sentence totally makes him seem powerful; he's ascending the stairs, coming close, and standing above her. Her reaction is to slink away. He is not only powerful here, but a little scary, too.
"Just that I see."
"You don't," she challenged. "Tell me what it is." (19-20)
Now, not only is the man above the woman on the stairs, but he's discovered her secret, and that's definitely a source of power in this poem. Now that he knows what she's so afraid of when she walks up and down the stairs, he can steer the conversation as he pleases. But the wife's not quite willing to give up the power yet. She doesn't believe that he's discovered her secret. Meanwhile, he's relishing his ability to torture her with his new knowledge. These lines are a little tug of war over that secret, and the power it gives the person who knows it.