I'm trying to understand too so please correct me if this is wrong. As I was able to gather it's like, you send them a camera which can only hold one picture.

I'm trying to understand too so please correct me if this is wrong. As I was able to gather it's like, you send them a camera which can only hold one picture. Then they use this camera to scan every document they have. The trick is that this camera will only save the picture, without revealing them which one, of the record you are interested in. When they finish scanning all the documents, they send you back the camera which now has a single picture of the document you wanted, and than only you can access.