It works the way all other forwarders work. Once the app detects an incoming message it simply sends the message to whatever number you set up.
On the second phone it comes up as a message from phone one. The message says “forwarded from 12345678” and then it displays the message.
Surprising enough I receive the message on the second phone before it even shows up in message plus on my exalt. Based off how slow message plus is (which needs a thread of it’s own) as apposed to this light weight forwarder.