ACARS Data Switched Off, Satcom still ON
The ACARS data feed from the engine systems (for Rolls Royce) and other aircraft systems (sent to Boeing) stopped or was switched off.
The Satcom system (used by ACARS as an alternative to VHF links to ground stations) continued to operate, but only exchanged routine information (pings) with Inmarsat to confirm that the link was still alive. (These "pings" contain information identifying the querying aircraft.)
This is like your Outlook email browser checking with an email server every few minutes. The message check proceeds even if no messages are waiting to be sent or received. Of course, the client needs to identify itself to enable the server to check for messages for that aircraft. Normally, the senders and recipients of messages never hear about these purely procedural link maintenance communications.
When the Malaysian government says that ACARS was "switched off", they mean only that no messages were sent or received after a certain time. Neither the Malaysian government nor the hijackers realized that the Satcom link would remain active even without ACARS messages to transmit or receive.