The reason it is called a smile drain is because of the way it looks. From above the drain actually resembles a smile. These drains usually follow the lowest elevation contour on the green. With the smile drain in it pulls the water that sits on the lower edge of the green. Typically all new USGA greens are constructed with smile drains. But for many years USGA greens didn't have smile drains so the fronts or lower elevations of USGA greens had similar problems.
Digging out the smile drain
Old Purr-Wick drains
the one to the left is not even connected?
Completed smile drain trench
See how wet the sand and soil are