Board Thread:Manga/@comment-5774380-20151009004044/@comment-26932251-20151009052710

Yeah, the wiki page needs to be corrected to be stated that it's unknown what that technology is or why Erwin looked at it when making that 'something to hide' comment.

Also, later, when he is shown shooting the flare halting the operation, that arm and that device is shown again, so I don't think Isayama goofed on the arms because I doubt they have prosthetics where the arm could flex and also have enough control in the hand to grab a flare gun and pull the trigger.

I suspect Ymir was the lookout because she was small and fast. We'll almost certainly find out next month.