"We're trying to create a little bit of different muscle memory," Williams said. "I think he's back to where he wants to be with regard to where his hands are when he starts [his swing], which will allow him to have a better path to the baseball.
"So today we're giving him a day because he's working extra. Today is a day for him to get that work done."
Desmond has struggled to a .219 average and .609 OPS and seven home runs. In 97 at-bats since the start of June, Desmond has hit just .155, while his strikeout rate has risen to a career-high 28.5 percent.
"He tends to be a little bit underneath [the ball] right now," Williams said. "So if his hands can stay a little bit higher in his approach to the baseball, then his plane's better. He doesn't foul as many balls off."
Desmond is hoping to get back into the form he showed during his previous three seasons, when he hit .275 with a .788 OPS with 69 home runs while the NL Silver Slugger Award each season at shortstop, in addition to making the 2012 NL All-Star team.
Williams talked about the difficulty of making mechanical adjustments mid-season while playing every day, and said he's hoping this day off can help Desmond establish the changes.
"During the season, he doesn't get a chance to [have multiple days off] because he plays every day, as well he should," Williams said. "His track record speaks for itself. There are occasions like today where we just have to give him the day off."
Carlos Collazo is an associate reporter for MLB.com.