https://www.cnet.com/news/lg-shows-twinning-is-winning-with-dual-screen-attachment-for-the-5g-v50-phone/

At first glance, the attachment looks like a folio phone case, but when you open it up like a book, the right hand inner cover where you'd usually clip your phone features an in-built display. Instead you click your phone into the left-hand side of the case, making sure that the three pins on the rear of the phone match up with the sensors inside the case. 

Two of the pins are used for power, and one for communication, although the phone also communicates with the attachment using a proprietary wireless technology designed for carrying heavy data loads over extremely short distances. The hinge on the attachment allows for a full 360-degree rotation, but is mainly designed to be used in two positions -- on an obtuse angle, or at 180 degrees so it can lie flat on a table.