How does PiP work physically?

[powersfoss]

As I understand it, we'll soon be getting picture-in-picture coming to our PS3 or standalone blu-ray players when the profile 1.1 firmware updates come to our players. So, the main movie is being read from a file on the BD and at the same time a second PiP file is being read from to display the PiP video. My question is, how does the player physically read from 2 different files on the disc at the same time? Doesn't the laser only hit one spot of the disc?

Someone care to enlighten me? Thanks.