The Preview FX option works the same in the Spark Editor as in every other editor and on the timeline. However, if the element to which you are applying the Sparks also has an Axis or a Wipe effect, you will not see the result of the Axis or Wipe effect in the Sparks module while using Preview FX because these soft effects are upstream of the Sparks effect. If you want to preview all effects, you must exit the Spark Editor and use the Preview FX option in the Player.