The second line is sung to this musical phrase:
In the first verse, the text here is "He opened death's prison, the incarnate, true Word" (with "the incarnate" elided together). The "opened" is sung with a melisma (the first syllable is sung to two notes; the word as a whole sung to three), and the first two notes form a fairly large interval of a fourth (A to D). Musically, this represents that opening.
In the fourth verse, the text here is "Christ rose, and now open is fair Eden's door." "Rose" is sung (with a melisma) to those same two notes, and the ascent musically represents that rising.
