Question: When fasting, we stop eating and drinking with the start of imsak time, but what is the reason for the need to perform Salat al-Fajr 15-20 minutes (depending on seasons) after imsak time?
ANSWER
The first fajr should, as a matter of precaution, be preferred for starting fasting and for the end of the time for Salat al-Isha. The second fajr is used for the start of the time for Salat al-Fajr. The same is written in Sharh al-Wikaya, too. (Hindiyya)

That is, one must perform Salat al-Isha by the start of the time of imsak at the latest. Similarly, one must start fasting at the time of imsak at the latest.

The time for Salat al-Fajr begins, in all the four madhhabs, at the end of canonical night, that is, with the sighting of the whiteness called fajr as-sadiq (true dawn) at one of the points on the line of ufq az-zahiri (apparent horizon) in the east. (Endless Bliss)