It was clarified at the press conference today, that they do not know when the ACARS was turn off/stopped. They only know that the last transmission was at 17.07 and that the next transmission expected 17.37 never happened.
We have three phases of the flight:
1) The known part which took MH370 to IGARI. Around 17.20 we had the last voice comm and transponder stopped. Last ACARS was 17.07 and next expected ACARS was 17.37. So all three types of transmission could have stooped/been disabled at the same time due to human action or technical malfunction.
2) The primary radar part where the flight is seen tracking towards VAMPI/GILVA/IGREX. We know little about that track. Important is if the aircraft could be seen to be manoeuvring in a way indicating humans controlling the flight path.
3) The 'dark' part where MH370 is travelling to its faith on the 40 dgr arc north or south. This could either have been under control of humans as intended by pilots/hijackers OR just a continuation of whatever was programmed at some stage before loss of control (HELIOS style).
Remember that what happened may not be what hijackers intended or the pilots tried to accomplish in an attempt to save the aircraft.