How does Swain control Sion?

As in what's his strategy beyond "Point Sion at enemy city, sit back, watch"? Of course getting Sion to wreck whoever or whatever's currently in front of him isn't an issue. But how does Swain get him to stop? Sion's bio says that hundreds of Noxians died just trying to restrain him back when Boram tried to get him reinterred. Getting Sion to stop was clearly an issue back in Boram's day, so much so that Boram stopped using Sion altogether.

Now Swain's using him as a siege engine. The bio mentions Swain brought Sion to Hvardis in an iron cage, but how in the world did he get him back in the cage, and then back inside his monument? Swain must have pulled it off somehow because the next few sentences talk about Sion seeing light come into his tomb like it's a recurring event. But it never says Swain found a way to stop Sion from killing his own allies or give him any kind of "off" switch.

Swain's got a bunch of arcane Secrets thanks to his demon arm and general intelligence network, so I'm sure he's figured out some way to get his suffering abomination of flesh under a degree of control. I just have no idea what that way is.