User blog comment:Darksusanoo/(Champion Rework) Syndra, the Dark Sovereign/@comment-1330314-20150728224700/@comment-1920550-20150728230857

Read the description....subsequent casts after the first within a 15 second window will only use the spheres already on the field, so after the first use, which will use the passive spheres and any ones you can cast, subsequent casts will make her "lose"  the passive spheres and only use the ones already on the field.