Misaḥḥ describes energetic, sustained running; it is a specialized equine descriptor here, not “ritual wiping” from discussions of purification. Al-sābiḥāt literally relates to swimming, but figuratively represents horses whose strides are fluid and rapid.
Idhā mā establishes the circumstance “when.” Atharna, from the root th-w-r (ث و ر), means they stirred up; al-ghubār is its direct object, dust. Bi-l-kadīd indicates the kind of ground on which the action occurs.
The shared feature between swimming and galloping is unbroken flowing movement. A literal picture of horses swimming in a river is inappropriate: the verse explicitly describes dust raised on land.