User blog comment:KnocknickKnock/Lucy Stardress:Crux Form/@comment-7418318-20150520191748/@comment-24264679-20150520195630

Crux magic is knowledge he is an expert on Celestial spirit magic. Lucy explained this multiple times throughout the series. When Crux looks like he is sleep he is searching the Celestial spirit Databases. That is Crux ability. Also Lucy gains the power of the celestial spirit she gained full control of Virgo's diver magic and Taurus strength. If Lucy was to use Crux star dress she would gain the ability to search the Celestial database. Crux don't right off the top of his mind know the information he has a database which is his magic. It not like she is gaining his memories or thoughts just access to the celestial database.