The comparative exposure at the beginning of the prayer prevents the validity of the prayer if the exposure is a quarter of the body or more; because covering the private parts is a condition for the validity of starting the prayer, and without it, the initiation is not valid at all. This is different from the exposure that occurs during the prayer. If someone's private parts are exposed while in prayer, and they immediately cover themselves, their prayer is not invalidated. However, it will be invalidated if a specified time passes, which is if they perform a pillar of the prayer in reality while exposed, according to Muhammad (may Allah have mercy on him). According to Abu Yusuf (may Allah have mercy on him), it is if a time passes in which it is possible to perform a pillar of the prayer – that is, the duration of three tasbihs. See: Maraqi al-Falah pp. 210-211, Al-Wiqayah and its explanation by Sadr al-Shari'ah 1: 143, Al-Radd al-Muhtar 1: 408, Bada'i al-Sana'i 1: 117, and Al-Hadiyyah al-Alaiyyah p. 72.