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.