وإذا احتبى قربوسه بعنانه
And when the front of his saddle, with its rein, seems to assume the posture of iḥtibāʾ.
The verb iḥtabā properly evokes a human adopting a particular bound sitting posture. Here it is attributed to the saddle pommel. The saddle does not literally gather its knees; the wording describes the visual pattern formed by the rein.
Both the unexpected attribution and the finely constructed resemblance make the expression a distinctive metaphor. The point is a recoverable but uncommon picture, not an arbitrary verbal puzzle.