Sentence:  As with the voice, this physicality is natural and flexible, a background of four-beat lines, grouped into four five-line stanzas (a central section flanked by anacrusis and <WSD>caudation</WSD>) with occasional moves to five- or six- beat lines, and occasional catalexis ("unrealized"/"silent" beats), always at stanzaic ends. 
Meaning of the word marked with <WSD> tags:  A section appended to the end of a word, line, or poem.