How to simulate instant funding for brokerage accounts?

I want to use the Broker API to simulate instant account funding for customers. I am wondering how that would actually work. It seems like this would be possible with journaling, but this is an unfamiliar concept to me. Is journaling just a way to instantly transfer assets between different accounts? So if i want to enable instant funding, and a user deposits $x.xx, I would journal $x.xx from an already funded account to their account and they would be able to instantly trade that asset?