Sentence:  Therefore, an <WSD>understep</WSD> (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. 
Meaning of the word marked with <WSD> tags:  A gait in which the hind feet touch the ground behind the point where the front feet touch the ground.