On win8 the App can go to "suspended" mode. to get messages when in suspended mode, we have to implement a background task and a way of keepalive "pinging".
Microsoft published some documents which explain the usage in detail. You should study and understand those documents first.
In MatriX there is a event OnBeforeRegisterControlChannelTrigger. There you can set RegisterControlChannelTrigger to true and pass optional stuff when required like the channel id or slot properties.
The default values are the following:
ChannelId = "MatriX_Channel";
ServerKeepAliveInterval = 30;
ControlChannelTriggerResourceType = ControlChannelTriggerResourceType.RequestHardwareSlot;
during the registration of the control channel it fires the event OnRegisterBackgroundTasks. This is where you can register background tasks with the newly created ControlChannelTrigger which you have access to in the EventArgs.