The speaker wonders where the cliché that "your life flash[es] before your eyes / while you drown" came from. Though the poem focuses on drowning in particular, the speaker's thoughts apply to any situation in which you know (or think) you're about to die. The speaker goes on to explain why this whole "life flash" idea seems so bizarre and discomforting.
For one thing, it suggests that "panic, or the act of submergence" could make time itself shrink—that the frantic terror of knowing you're about to die, or the mere fact of suddenly being under water, had the power to "startle time into such compression." The figurative language here treats time like an animal recoiling in shock or fear at a sudden noise.
Digging into the cliché even deeper, the speaker presents this flash as downright violent when taken literally. It would entail an entire lifetime getting squashed into a few horrifying moments—or, as the speaker puts it, "decades" would get crushed "in the vice of your desperate, final seconds." This metaphor compares the "desperate, final seconds" of your life to a tool used to hold objects firmly in place, like a clamp. The image conveys the painful crunch of time collapsing on itself, as though your whole life were being crudely smooshed in a fist. Basically, the speaker argues that experiencing a whole life all at once would be frenetic and overwhelming—not exactly a pleasant way to go.
The sounds of the poem enhance the speaker's argument. Take lines 3-5, which are filled with sharp /c/ alliteration and consonance and hissing sibilance:
[...] as if panic, or the act of submergence,
could startle time into such compression, crushing
decades in the vice of your desperate, final seconds.
These crunching, spitting sounds convey just how frightening and uncomfortable it would be for your life to flash before your eyes as you die. Surely, implies the poem, people would prefer a more serene, calming experience.
Despite the subject at hand, the tone of this stanza, and the poem as a whole, remains lighthearted and whimsical. It sounds conversational, opening with a simple "I wonder," and referring to the "life flash" colloquially as "this business." Billy Collins's poems are often tender and funny. Here, the casual tone also supports the speaker's argument that people treat both death and life with too much seriousness and self-importance. The use of free verse—that is, the lack of a regular meter or rhyme scheme—adds to the speaker's humor, making the poem sound more off-the-cuff.