R. Yonah: This teaches a greater Gevurah than was mentioned above. a Chacham rules over his desire - "u'Moshel b'Rucho mi'Loched Ir" (16:32). Chachmah gives power to the Nefesh to rule over desires - "Lishmarcha me'Ishah Zarah", "Lishmarcha me'Eshes Ra" (7:5, 6:24). He guards his mouth from bad foods - this is a Midah in separation. One should eat only to sustain the body, and not be drawn after pleasures. Therefore, he abandons excess food and bad foods.

Malbim: However, there is a condition - he must beware that the conquered not return to conquer the city. He must guard his mouth and tongue - the fortress of Chachmah, the Nefesh that speaks. Via there the enemies enter to besiege the Nefesh. Guarding speech can be like the simple meaning. Or, the mouth hints to speech of Chachmah and the tongue hints to speech of Tevunah, which one must guard, not to stray from the way of Chachmah and Binah. In this way, he guards his spiritual Nefesh from afflictions, lest the enemies that constantly fight against it besiege it, when it destroys the wall and the one who covered it is revealed.