Well, first you have to watch the 2nd season of Code Geass if you haven't.
As much as I understood, this is happening after
SPOILER IF YOU HAVEN'T WATCHED THE 2ND SEASON:
.
.
.
.
.
Lelouch died (but he ended up alive?, but anyways). Before he died, wearing his Zero mask, he used his geass on Schneizel. He said that he has to do whatever he says or something like that. After Lelouch died, Suzaku became Zero.
So, in this story, Suzaku wants to "punish" Schneizel for some reason (Because he was a bad guy? Or maybe because he was Britannian? But mainly just for fun I guess), and that's why he is able to control Schneizel but only when he wears the mask (once he took it off, Schneizel realized what was happening.). He is lying to Schneizel's sister - Cornelia, that he will stop the geass from working which means that Schneizel will be free from the "curse", always saying that "This will end soon" or something like that.
I hope I explained it! ^_^
Oh, and.. The thing that Schneizel was wearing is a prison uniform in their world, which also makes it, I think, impossible to move, so he doesn't run away.
Someone please explain this to me. I don’t understand the dynamic or why this is happening.