Here's one way to think about it. Imagine you can see an object traveling above the speed of light. It creates a streak across your vision because it appears in several spaces simultaneously. In the direction of travel, the object crashes into a rock. Normally, there is no way to prevent the accident, because the information could only move at the speed of light, slower than the apparent velocity of the object. In "subspace," where normal spacetime is not in play, it would be theoretically possible for you to observe the crash and communicate the information to the ship to avoid the crash. The causal order would be reversed--from the perspective of normal space.
According to DS9, the Prophets live in a limited area of subspace. They are not confined by the normal causal order. However, a cause must still take place in order for the effect to precede it. They learn about Ben Sisko and intend to use him to communicate to the Bajorans. That knowledge permeats their society, which extends temporally. They communicate messages about what Sisko should do, but that information appears and many different points on the timeline. The Prophets become concerned that someone will interfere with Sisko's development, to they send one of their own to insure his birth. That Prophet is able to access Sarah Sisko's consciousness so that she can act in a way consistent with her normal behavior (as the Pahwraith did in The Assignment). By doing so, the effect of meeting Sisko becomes the cause that creates (or insures) his birth.
