The simple understanding of the Gemara is like Rashi that he causes the animal not to eat by his voice by yelling at the animal when it is about to eat.
However, if you look carefully at Tosafos (ד"ה רבי יוחנן), you will see that Tosafos disagrees. Tosafos is bothered by a question that רבי יוחנן himself holds that עקימת שפתיו is not a מעשה by נשבע מימר ומקלל and therefore what is the חילוק here and why should חסימה בקול be חייב? Tosafos answers that here he is causing the animal to be דש with his voice and therefore בדיבוריה קא עביד מעשה. Tosafos learns pshat in חסמה בקול that he muzzled the animal normally and then with his voice caused the animal to be דש. This is very difficult in the words of the Gemara, according to Tosafos the Gemara should have asked דשה בקול. He wasn't חוסם בקול here at all. Tosafos is forced into this pshat because they need his דיבור to actually do something, cause the עבירה to happen. By לא תחסם the עיקר עבירה is the דישה while the animal is muzzled and therefore they had to learn that his דיבור caused the דישה.