The reader sends energy to the tags but the tags sit idle until the reader requests them to respond.
The reader is able to find tags with specific serial numbers by asking all tags with a serial number that starts with either 1 or 0 to respond.
If more than one responds, the reader might ask for all tags with a serial number that starts with 01 to respond, and then 010.
This is called "walking" a binary tree, or "tree walking".
See Singulation for additional relevant information.