[…] dapple-dawn-drawn Falcon (2)
This metaphor makes it seem like the windhover is being "drawn" by the "dawn" the way a carriage is drawn by a horse. It's kind of a funny mental image—like the bird is being pulled by the dawn.
[…] he rung upon the rein of a wimpling wing (4)
Whoa. Now the bird is like a powerful horseback rider, and his wings are the reins he's using to harness the power of the wind. It's another unexpected mental image that makes us see the bird in a new, more exciting way.
[…] then off, off forth on swing, As a skate's heel sweeps smooth on a bow-bend: […] (5-6)
The bird changes yet again, with another metaphor: now the bird appears to be an ice skater, suddenly gliding in an arc, or a "bend."