User blog comment:XGlass Reflection/Ep. 25 Review~ I'M ALL FIRED UP/@comment-32788838-20131004002212

I'm seeing a lot of hate about Eren's Titan's "Second Form" or whatever you people want to call it, though I honestly don't see why you should because, as far as I know, this isn't a second form or anything like that.

Best explanation I can give is that Eren's anger and immersion into being a Titan allowed some form of internal chemical reaction to occur which, as a side effect, enhanced and increased the rate of Eren's regeneration. Annie does something similar earlier in the series with focussing her regenerative ability, perhaps Eren does something similar with a much better approach towards it?