Ah, yes! Sorry if that was unclear. In canon, Reiner injures his arm saving his friend, Connie, in Castle Utgard. Just as a Titan is about to lunge at Connie from the castle's basement, Reiner steps in the way, pushing Connie out of the Titan's reach but putting himself in harm's way. The Titan pretty much chomps down on Reiner's right arm, fracturing it and leaving some pretty nasty bite marks that don't heal until... well, the big reveal when Reiner consciously heals it to show Eren that, "Hey! Guess what, I'm the Armored Titan!" This wound is important because it is the most solid proof of the fact that Reiner was 'going native' and risking his life for his human friends beyond what was rational. (For his reincarnation though, I just wanted the action to echo more than the emotion.)

I changed a couple of the details as far as the echo goes, but that's the general idea. Does that make sense? It'd be cool if he made it disappear once he got his memories back hahahahahahaha