Tafsir of Al-Adiyat 100:2

Al-Ira' (igniting) is the bringing forth of fire, and al-qadh (striking) is the well-known act of striking and clashing. It is said, "He struck (qadaha) and it ignited (fawra)," if he brought forth fire; and "He struck (qadaha) and it failed to spark (aslada)," if he struck it but it did not bring forth fire.

The intended meaning here is also the horses; that is, those that ignite fire through the clashing of their hooves against stones. This fire is called nar al-habahib (the fire of fireflies), which is the name of a stingy man who would only kindle a weak fire for fear of guests; it became a proverb, until they applied it to that which is ignited by the hooves of horses and the feet of camels.

The accusative case of qadhan is like that of dhabhan—as mentioned previously—being a case of tamyiz (specifying/discriminatory noun) transformed from the agent; that is: "those whose striking ignites." It is also said it refers to [the horses themselves] acting as agents, though this is further removed from the [meaning of] qadh. According to Qatadah, al-muriyat is a metaphor for horses that ignite the fire of war and kindle it, but this is contrary to the apparent meaning.