I've been confused about Imsak since forever. So here is what I found:
The time for fasting commences at Subah Sadiq (true dawn). That is also the commencing time for Fajr Salat. The ten minutes is to exercise precaution to avoid eating after Subah Sadiq. If one is sure of the time he may eat till subah sadiq. And Allah Ta'ala Knows Best
Imsak time: It is the time at which fasting starts (e.g. during Ramadan). The timing of Imsak time is not well defined and often, it is related to the timing of Fajr. However, since Fajr time can not be calculated exactly (only approximated time), most Muslim countries differ between Fajr and Imsak time by 5-10 minutes.
...There is no time before dawn called "Imsak". What is found in some leaflets and diaries specifying a short time before the Fajr Adhan as the starting time of fasting is a precaution only. There is no evidence for this. In fact, it is contrary to the Sunnah. Indeed, it is lawful for us to eat, drink etc. from sunset to dawn. Whenever dawn appears, we start fasting and stop doing all things that are not allowed during fasting.
Allah knows best.
...Pemberitahuan imsak ini memang sangat diperlukan karena akan sangat sulit bagi kita untuk berhenti dari makan misalnya seketika saat mendengar adzan Shubuh. Jadi misal saja jarak antara imsak dan adzan Shubuh itu sekitar 10 menit, padahal nasi di piring tinggal 2 cendok, tak apa-apa habiskan saja 2 cendok itu lantas segera minum dan membersihkan mulut dari sisa-sisa makanan. Karena waktu 10 menit itu cukup untuk menghabiskan 2 sendok, minum dan membersihkan mulut.
Err, I was expecting something from the Malaysian Jabatan Agama Islam (be it the state level or national), but all I found was the above links (and several others). And as usual, I think I don't know how to google. :(
**A little (or rather important) update here**
Cik Yan has found this.