The third verse ends with the lines "Let the valleys rise to meet Him / And the hills bow down to greet Him," sung to these phrases from the tune "Freu dich sehr":
The second phrase generally descends, illustrating the hills' "bow[ing] down."
The other thing I noticed is a feature in the text: "far and near" in the lines "Hark, the herald's voice is crying / In the desert far and near" at the beginning of the third verse is a merism.