On days when birds are passing upwind out of earshot or when you are running traffic fields, flagging might turn birds that won't hear your calling or might commit quickly to another field.
Sometimes, especially with early-season geese, flagging bring them right back around for another pass.
Although flagging is mainly associated with field hunting for geese, don't forget about ducks.
Bluebills, redheads and scoters all respond to flagging motion that emulates landing birds in open water.
As with calling, practice can make a difference when it comes to flagging motion.