Hopkins's "spring rythm" method was developed as an alternative to iambic pentameter. Hopkins wanted to avoid what he thought of as the "sing song" quality of the iamb and find a meter that was more in keeping with real speech. Hopkins relied on the spondee as the basis for this new metrical form: a spondee is two accented syllables together.

In the lines of "Windhover" you mention you can see this emphasis on the stressed syllable (stressed syllables in bold).

In his ecs ta sy! then off, off forth on swing,
As a skate’s heel sweeps smooth on a bow-bend: the hurl and glid ing
Re buffed the big wind. My heart in hid ing
Stirred for a bird,—the a chieve of, the mas ter y of the thing!

You can easily see how prominent the stressed syllables are, and when you read this aloud, all the stresses give the language a kind of "rocky" feel. It's difficult to say out loud, but the sound of the poem is meant to evoke the image the words represent—seeing a falcon, wings beating against the wind, then veering off and gliding effortlessly with the wind.

The poet's emotional response to the bird's "mastery" is also contained in this stress-heavy language, especially in the surprising rhyme "stirred for a bird." This is surprising because of its directness, compared to the rest of the poem, and because the rhyme serves as a kind of emphasis, a way of underlining an emotional fact.