Answered by Sidi Faraz Rabbani from the Hanafi Fiqh List
Most books of fiqh seem to state that the wajib time to begin fasting is at the true dawn (when the second adhan in pronounced). I have noticed that many Hanafi fasting calendars (and the calendars in Shafi'i Malaysia) require people to stop fasting 10 minutes before the true dawn. What is the reason for this? Is it simply a way of protecting the fasters from falling into the true dawn?
This is not a ‘requirement’ but, rather, a recommendation, for the reason you mentioned.
It is mentioned in Bukhari and Muslim, from Zaid ibn Thabit, “We had the pre-dawn meal with the Messenger of Allah (Allah bless him and give him peace), and then we got up to pray." The sub-narrator said, 'I asked, "How much time was there between the two?" He [Zaid] said, "50 verses."
There are other hadiths about this. But they do not indicate obligation. Rather, recommendation, by agreement, with the reasoning being not to enter a time in which there is doubt whether fajr has come in or not. [Nawawi, Majmu` 6.406; Ibn Qudama, Mughni 3.2127; Ibn al-Humam, Fath al-Qadir 2.374-375; Ibn Abidin, Radd al-Muhtar; Buhuti, Kashshaf al-Qina` 2.331]