He the reason why loud recitation (Jahr) is prescribed in some prayers and not in others is explained. The prayers in which recitation is loud are done during dark times, so it is necessary to recite loudly so that people passing by can know that a congregation is present. If they want to pray, they can join. Without loud recitation, it would be hard for those passing by to know there is a congregation. The prayers in which recitation is not loud are performed in the daytime when it's bright, so they can be known through sight, and there is no need for hearing. When you recite the Al-Fatiha and a Surah, you should say Takbir once while standing, then bow and place your right hand on your right knee before your left, with your palms on your knees. Spread your fingers around the knee joint, and extend your neck, keeping your gaze directed between your feet, toward the place of prostration."
Commentary #6