I would assume - since one is logged in already - to wikitree and potentially the external site ... let's say familysearch (where I do most of my searching now), that it's just pulling that visible data into the form. If you are logged in you would already be viewing it if you had the rights (trusted list or manager) - it wouldn't breach security and doesn't transfer the data to the other site, it just gets it read for you to search the other site.