The metaphors in "Introduction to Poetry" propose creative, lively ways to approach poems—and contrast them with the deadening analytical methods that students often learn.
For example, the speaker urges students to listen closely to the inner activity of a given poem, telling them to "press an ear against its hive." This metaphor presents the poem as a beehive, something that is full of life—and something that just plain sounds good. Telling students to "press an ear against" the "hive" of the poem encourages them to simply listen to it: by listening, the speaker implies, they'll get a feeling for the poem's nature, learning in a deeper way than they could through dry analysis.
Other metaphors in "Introduction to Poetry" frame reading poetry as an act of exploration. This is the case when the speaker tells students to "drop a mouse into the poem / and watch him probe his way out." Similarly, the speaker says that students should "walk inside the poem's room / and feel the walls for a light switch." In these metaphors, poems becomes mazes and darkened rooms: strange, unknown environments to be slowly explored, not instantly solved or illuminated.
Next, the speaker says that he wants his students to "waterski / across the surface of a poem." This presents the act of reading poetry not just as an exploration, but as an exhilarating adventure. Poetry, the speaker implies, can be fun. This metaphor also suggests that sometimes it's enough to skim across the "surface" of a poem, enjoying it for its most obvious pleasures rather than diving into the deep, murky waters of analysis.
This is the exact idea that the poem's final metaphor hints at. When the speaker says that students always want to take a poem hostage and "torture a confession out of it," he implies that people often try to squeeze meaning out of a poem, assuming there's some kind of hidden secret that will help them suddenly make sense of it in a more profound way. The speaker's students "beat" the poem "with a hose / to find out what it really means"—a graphic image that suggests joyless analysis can completely ruin the fun of reading poetry.