Impasto is one of those fancy art terms, and it means slapping on the paint really thick. There are several ways this can be done; using a palette knife will do the job, but I think the results are usually pretty ugly -- messy-looking. Almost any kind of large brush will do the job too, but one with a squared-off end easily signals impasto!, impasto!!. And thanks to this brush effect, you don't have to lay on paint so thickly to get an impasto appearance.
Some artists have built reputations around their use of square brushes (though they seldom make an entire painting using such a tool). Over-use of square brushes can yield results just as off-putting as palette knife painting, so moderation is usually a smart strategy.
Let's look at some works by artists who made use of square brushes. Click on images to enlarge.
Flohr is currently active, and giclées of his paintings can be found in many art galleries around the USA. He uses square brushes much of the time, yielding a signature look that probably assisted his career. I find some of his work interesting, but his technique mostly seems to get in the way of what he's trying to depict. Perhaps he's already evolving from so much reliance on square brushes; I hope so, anyway. As for the painting above, I think that there are too many brush strokes of similar width and length; more variety in strokes (not to mention use of brushes with other-shaped tips) would have improved it.
If it weren't for John the Baptist's head on that plate, this would be simply an interesting nude-in-the-woods painting. I like the use of warm and cool colors applied in large patches by square brushes; a little extreme, maybe, but it gives the work its unique character.
During the early years of the 20th century Putz made a number of paintings featuring areas painted using square brushes. The result is a faceted look which, while mannered, intrigues me as an artist (of sorts).
Up to the early 1940s Mead Schaeffer created illustrations using a strong, "painterly" style where brush strokes were often obvious even when seen on printed pages. The illustration above is one of his best, and used square brushwork in certain places such as the lady's gown, but not all over.
Tepper worked in a painterly style around the same time Schaeffer did and also created many fine illustrations. Square brushwork in this example can be seen on parts of the building's wall as well as on some of the rubble.
Manchess is currently active as an illustrator of science fiction and fantasy book covers and he also paints murals and does other commercial illustration. He has a strong, painterly style and isn't afraid to use a square brush in places, as the example above indicates.