They are singing not a religious qawwali but its secular and lay cousin.

I guessed, rather unconfidently.

Dan Chaucer, well of English vndefyled, / On Fames eternall beadroll worthie to be fyled.

Some authorities profess to discover jazeraint work in this representation, which was a method of defence much used in later centuries for archers' jacques and various other garments, but we have no right to assume that the Normans at that period carried such a heavy weight of armor as this would necessitate, or were acquainted with such a technical and complicated manufacture as jazeraint work.