The Surface Book features a "Muscle Wire Lock," that will keep your display securely fastened to the base, until it's time to undock and get more mobile.

In this case, electricity is run through the wire, heating it up and causing it to contract. This pulls the black pulley inward, against the spring, lifting the lower arm of the linkage.

That lower arm holds a very tiny rod captive. The rod serves as the grip, that holds onto the base unit's metal tab. When the linkage is retracted, the rod rolls out of the way and lets the base go.