Trochee trips from long to short; (1)
The word "trips" perfectly describes the trochee. The movement from something long to something short, or from something stressed to something less stressed, is kind of like a little stumble.
From long to long in solemn sortSlow Spondee stalks, strong foot! […] (2-3)
Notice how many times the letter "s" occurs in these lines (solemn, sort, slow, spondee, stalks, strong). The repetition of the letter in six straight words mimics the repetitive nature of the spondee, which contains two long syllables in a row. The lines themselves are monotonous and slow, just like the spondee.
Iambics march from short to long. (5)
The speaker says, "iambics march." The word "march" reminds us of things that are highly regimented, like armies, marching bands, that sort of thing. The speaker implies that the iamb is very regular, almost levelheaded. It's just right – neither too fast nor too slow. Is this a key to the prevalence of the iamb in English poetry?