In "Birches," the author uses the line "like girls on hands and knees that throw their hair before them over their heads to dry in the sun" to describe

The shape of the trees after an ice storm

Explanation:

In  "Birches," the author uses the line "like girls on hands and knees that throw their hair before them over their heads to dry in the sun" to describe how the shape of the trees after an ice storm.

This figurative speech further explains the role participated by the tree i.e

The trees bend slightly like the girls and dry their hair under the sun.

the shape of trees after an ice storm

Explanation:

the author uses the line "like girls on hands and knees that throw their hair before them over their heads to dry in the sun" to describe how the shape of the trees after an ice storm