A gait in which the hind foot touches ground in front of where the front foot touches the ground.
A few animals, such as bison, commonly use an understep walk, while several species, including black bears, cougars and pronghorn regularly use an overstep walk.
Therefore, an understep (where the hind track lies behind the front track) is probably a slower gait than a direct-registering walk where the hind lies on top of the front, and both are probably slower than an overstep walk, where the hind track registers beyond the front track.