Which word does muwaṣṣal describe in bi-khayṭin muwaṣṣal?

Imruʾ al-Qays: The Horse’s Speed, Agility, and Similes (Verses 52–58)

• Al-walīd: the child
• Khayṭ: an attached or connected string
• Kumayt: the coat color
• Al-sayl: the torrent