I am having trouble understanding and believing. How did you get an ostensible power terminal (for pull up) and two terminals for MiTM (input and output) from

I am having trouble understanding and believing. How did you get an ostensible power terminal (for pull up) and two terminals for MiTM (input and output) from two terminals? Assuming a situation where there are other pullups on the wire, how did you assert the low state (short to ground) without a connection to ground?