No complex mechanisms. It is a hand that can be seen on the caseback side, and makes a full revolution once every 366 days. It simply points to a disc which has properly spaced, varying month lengths between 29-31 days.
The reason this was done is because it conveniently ties in with another complication - the Equation of Time, which is an annual complication. Another watch that uses a fairly similar setup would be the Vianney Halter Classic Janvier, also on the caseback.
Regards,
skyeriding