In operational terms nothing happens, it's business as usual. 

Your application will still be able to send messages through Realtime but you'll be billed with an overage usage of $1 per each million messages.

I'll give you an example:
Let's say you are using the $30 USD basic Ignite plan.

If in a given month you send 40 million messages your billing for that month will be: 

$30 USD + 9 million messages in overage * $1 USD = $39 USD

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