Although we don't know the exact technical details from Inmarsat, we know a few things.
1. The initial projections of the flight path were based on ACARS Classic Aero ping return timings and or signal strength.
2. Ping timings and or signal strength are therefore logged.
3. The ping return timings and or signal strength gave an approximate flight path corridor in a north or south direction (but not which one).
4. The two possible approximate flight path corridors, north or south, don't seem to be in much dispute.
5. The Doppler Effect changes apparent frequency.
6. Therefore ping timings and or signal strength
and received frequency are logged.
7. The Doppler Effect could show if the aircraft was moving away from or towards the satellite and at what speed.
8. If the aircraft turned, the Doppler Effect could be used to see that an aircraft moving in one direction was now moving in another direction.
9. The possible tracks are known and that the aircraft turned is known. The following is from the Ministry of Transport Malaysia: