This exercise in sensory exaggeration often provides the necessary boost to reach the hole. If they're still leaving shots short, I ask them to envision a silo or grain elevator in place of the flag and to imagine their ball flying in the top and funneling straight down to the hole. These images often do the trick. In my experience, most amateurs try to guide the club through impact instead of letting it swing freely with maximum speed. The result is a major power shortage.
Some teachers go so far as to have students throw clubs down the range.
Feel free to create an image of your own to help call up the right action. When I catch myself guiding the club, I recall a game we played as kids. We used to stick hedge apples on long bamboo shoots and see who could fling them the farthest. To get much distance, you had to start slow and spring the stick at just the right spot, like a cowboy cracking a whip. This image helps me let the club free-wheel through impact.