When the plane ran out of fuel, the engine would stop. I think that's what the "partial ping" registered. What else would prompt a ping from 370? The engines were the reason that the pings were being transmitted, by Rolls Royce's design, to gauge engine performance. Some event triggered a "non-routine," i.e., non-hourly out of cycle, ping. When you consider miles traveled, fuel levels, etc., it seems this was the end of the line for the engines.
This is so important because it narrows down when and where the plane may have gone down. It's possible, per the pilots I have listened to, that the 777 could have then glided subsequent to the engines stopping, so they still have to consider gliding capacity and distance. But it still eliminates much of the original search area by fixing the engine shutdown at 8 minutes after the prior ping.
It's a nightmare though, that the currents and winds are adding a huge amount of variability into the search.
