I got the following info from Alan Walls (developer of IGCdroid), which may help explain how it actually works.

From Alan Walls (posted by his permission):

You can use your purchased IGCDroid on any number of devices that use the same Google Play account.

Regarding the cellular data usage, it isn't much. Typically IGCDroid does only send data to Glideport and configured email notifications. Map data is only downloaded if you choose to view the flight on the map.

To be more technical, IGCDroid literally sends the IGC data to Glideport, split into time sliced blocks. The transmission protocol probably adds 10-20% overhead on top of the base IGC data. I don't have exact figures on this.

The "live" aspect is a bit tricky because it depends more on cell phone network availability. This will never be as good as the Satellite options, but then its dramatically cheaper.

When logging is active IGCDroid is constantly recording GPS positions and logging them to the flight IGC file. This only pauses if the phone isn't getting GPS positions for any reason. This data recording rate is selectable in the app. I suggest using the "variable logging rate" (default) as this records more points when you are circling and less as you are flying straight.

In ideal cell coverage IGCDroid will send data to Glideport once a minute. This rate was chosen as a reasonable update rate that doesn't drain your phones batteries by constantly transmitting. At each transmission IGCDroid send all data that hasn't been sent since the last transmission. Data is never sent more than once.

When coverage is erratic, IGCDroid retries repeatedly for a few seconds then sleeps for another minute. This is to avoid draining the battery when out of cell coverage.

If you are interested to see when IGCDroid has sent data you can view the flight in the Google Map view. This view has little blue dots at each point data was sent.