Four lean hounds crouched low and smiling my heart fell dead before.
Woah. We sure didn't see that one coming. Remember all the times that the speaker slowly moves his allegiances away from the rider and toward the deer? Well, now we know why. The real target was the speaker's heart. The whole poem is an elaborate metaphor for the speaker falling in love with someone.
You know how people describe falling in love as being "hit by Cupid's arrow"? Well, our speaker's heart is (metaphorically) hit by his love's arrow.
Following that logic backwards into the poem, we discover that the speaker figures himself as the deer. (Could that be why he spends so much time describing them in beautiful detail?) And the huntress, who seems to be the protagonist of this account, is actually the antagonist—the one who knocks our speaker-deer to the ground.
Two quick points here: 1. Cummings uses a major pun as the punch line of the poem. "Hart" is another word for deer. And "heart" is what the rider, apparently, was stalking all along. The hunt is a pretty classical metaphor for chasing after someone. Remember Pepe le Pew? He's part of a very, very long tradition of love hunters. 2. The poem hinges on repetition in order to smack us with a major change. We thought we were reading about a hunter out on a hunt, which is exactly why the exact repetition of lines 1-3 makes us think that we've come to a neat ending. But we were really reading the story of an elaborate metaphor for falling in love—and suffering the consequences. Ouch.