The shape does the work
Two syllables barely fit a noun and an article. Four fit a small prepositional phrase. Six and eight fit a clause with a subject and a verb. Two again fit one word, or a word and a name. So the sequence is: thing, setting, action, complication, verdict.
The specimen follows exactly that: black ice (thing), on the whole path (setting), and my father's hand held (action), out behind him, not looking back (complication), for me (verdict). The last line is two syllables and it is the only line in the poem that is not about the ice. That is the snap.
Why it is often taught badly
School versions of the cinquain usually come with a required grammar: line 1 a noun, line 2 two adjectives, line 3 three verbs, line 4 a sentence, line 5 a synonym of line 1. That is not the form. It is a worksheet built on top of the form, and it produces five lines that say the same thing five ways.
The original form, as published by Adelaide Crapsey in 1915, specifies only the syllable count and a preference for the short final line. Everything else is yours. Keep the count; throw away the part-of-speech recipe.
Counting, and where the count can flex
Count syllables as they are actually spoken, not as they are spelled. Black ice is two. Whole is one. Father's is two. Counting the specimen: 2, 4, 6, 8, 2 — it lands exactly. If a line comes out one syllable long and reads better that way, keep it and accept that the poem is a near-cinquain; Crapsey's own drafts bend the count more often than the published versions suggest.