On a guitar with a stop tailpiece, threading the strings through the front (bridge side) of the tailpiece, so they exit through the rear of the tailpiece. The strings are then pulled (or wrapped) over the top of the tailpiece to pass them over the bridge.
This results in reduced string tension, and, according to some players, changes the sustain and tonal characteristics of the instrument. Similar results may be attained by raising the tailpiece up higher, but some players prefer to have the tailpiece screwed down tight against the body as they feel it improves the string/body vibration transfer, while still bringing the strings across the bridge at a lower break angle.