British satellite firm Inmarsat discovered on March 9 that MH370 had continued flying for at least another six hours after its last voice transmission from the cockpit at 1.19am on Saturday March 8.
It knew this from an electronic 'handshake' that the plane gave to one of its satellites. This placed it in one of two corridors, running north to kazakhstan, or south to the Indian Ocean.
The satellite received several more pings from the aircraft, but Inmarsat weren't immediately able to work out its trajectory because the satellite isn't GPS enabled.
However, the pings contained hidden clues in their wave frequencies. Inmarsat, using some very clever mathematical modelling, deduced that the frequency of the waves were changing in a way that gave away the plane's locations.
It's called the Doppler effect, named after the Austrian physicist Christian Doppler, who proposed it in 1842.
He noticed how wave frequencies changed as an observer moved around the source. In modern life we experience it frequently as the sounds of sirens from emergency service vehicles change as they move away from us.
A similar effect was occurring with MH370 that enabled Inmarsat to rule out a northerly trajectory. The frequencies of the pings it was returning meant it was travelling south.