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.
She withdrew shrinking from beneath his arm
That rested on the banister, and slid downstairs; (33-34)
Now that her secret has been found out, the woman's physical position has to match her position of power, so she slides down the stairs, below her husband. Imagine her power in the relationship doing a similar slide, shrinking down the bottom.
Her fingers moved the latch for all reply. (47)
While the woman may have lost her position on the staircase, she's still got some hope left. Now she's found a new power—the power to escape. She is close to the door, close to fleeing this fight, this man, and this house. Her husband doesn't want this to happen, which gives her power over him.
"I'll follow and bring you back by force. I will!—" (120)
Now, in the very last line of the poem, we jump from who has the power in the relationship and in this conversation, to physical power. In the end, as a man, who, as he has shown by digging his son's grave, is very strong, he has more physical force and power than the woman. No matter how much power she has in their verbal relationship, he has all the power physically. He really can drag her back into the house if she runs away. Here's hoping that's not how this goes down.