What is al-wishāḥ al-mufaṣṣal in the simile?

Imruʾ al-Qays: A Guarded Tent, the Pleiades, Objection, and Dunes (Verses 23–29)

• A guard’s sword as the tenor
• A ditch as the simile particle
• The beloved’s tent as direct object
• A jeweled sash serving as the comparison image