Tafsir of Fussilat 41:20

"Until, when they reach it" (i.e., the Fire, as the culmination of the gathering or the herding), meaning: until they arrive at it. The particle 'ma' is pleonastic (extra), intended to emphasize the immediate connection between the witnessing and the arrival, as it emphasizes that to which it is appended; thus, it reinforces the meaning of 'idha' (when). 'Idha' signifies the immediate connection of the response, as both occur at the same time—a matter which has no relation to grammar such that it would be problematic if grammarians failed to mention it, as Abu Hayyan has disparagingly claimed, for they deny it.

There is an ellipsis in the discourse; the estimate is: "Until, when they reach it and are asked about what they committed, and they deny it, then their hearing, their sights, and their skins bear witness against them concerning what they used to do." The mention of the witnessing suffices in place of the omitted portion, as the witnessing necessitates it. The estimation does not reject the emphasis of connection, since the occurrence of these events in a single assembly is sufficient for connection.

The apparent meaning is that the "skins" are the known ones. It is said: they are the limbs, used as a metonymy for them. It is also said: it is a metonymy for the private parts; it is stated that this is the view of the majority of exegetes, among them Ibn Abbas (may Allah be pleased with them both). In al-Irshad, it is noted that this is the most appropriate interpretation given the specificity of the questioning in His (the Exalted's) saying...