he who controls the exit nodes, controls the traffic

He who controls the exit nodes, controls the traffic.Mr. Robot, Elliot confronts the owner of Ron's Coffee. 

Tor exit nodes are the last “hops” that Tor network traffic goes through when making a circuit around it.

There is someone who operates each of the nodes on Tor (usually independent volunteers). However, some of these are operated by law enforcement agencies (like the NSA, for instance), or by other individuals who have the intent of spying on Tor traffic.

Exit node traffic is unencrypted, so in essence, the operator can analyze this traffic to try to find patterns and, in turn, potentially identify someone by their habits (at least that’s a simplified explanation).

Therefore, the more exit nodes someone operates, the better chance they have of collecting data about specific people on the network. Granted, this is not an overnight process, but over time, this has enabled certain organizations to be able to make arrests, or perhaps hack a specified target, as just two examples.

You might say, “He who controls the exit nodes controls Tor.”