Waterfowl hunters should use the highball selectively-at certain times and in certain places. When a front has passed and new birds are moving into the area, a series of highballs can mesmerize migratory flocks looking for a safe place to rest. When ducks are returning from feeding to a resting area, a well-timed highball might convince them to short-stop at your spread. If ducks are trading over a marsh or lake at long distance, you may need to call a lot to be heard a little.
Highball Calling Tutorials: (mp3s)
Highball: Part 1 | Highball: Part 2
In all these cases, when one or more ducks show signs of listening-slowing their wingbeats and perhaps sliding out to the side of the flock-pour the calling on. If a single breaks off and cups its wings, others will probably do likewise.
Conversely, blowing a highball is not the best approach on warm, still, overcast days; in the close confines of flooded timber or small marshes; or for "local ducks" that have been educated by heavy hunting pressure.
Related: 8 Duck Calls Every Hunter Must Master