"Whereas a normal text message would be routed to the recipient's text message inbox, a Flash SMS will display on the screen of the recipient's mobile device."
Found here [
^] via a simple Google search, which you could have done yourself within seconds. This answers the first part of you question ("Whats the flash sms...").
Now have a look
here[
^], maybe it will help you.
But why do you want to know, how to do it at all, if you don't know what it is?
modified:
Another link, which might be useful.[
^] The rest is up to you.
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