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."
[… ] the fire that breaks from thee then […] (10)
As the bird makes a dive toward the ground (he must have spotted a mouse or some other small animal to catch), the red on his chest flashes like fire. But the "fire that breaks from him" sounds kind of dangerous, like he's a falling star or a crashing airplane or something.
[…] shéer plód makes plough down sillion Shine […] (12-13)
The speaker says that even a simple plough is beautiful—when it's well used, the steel of it gets all shiny. The speaker seems to be suggesting that even well-worn everyday things, like farm tools, can appear beautiful if you look at them in the right way.