Actually at one point just before Reiner made his charge, there was a slight animation of armor breaking and being shed. I'm pretty sure that was to indicate the joint armor being removed to increase agility, at the exchange of opening weak spots. Before that, you notice you never saw him move quickly. In fact he seemed to purposely eat all Eren's blows in order to hit him back harder.
Until that point I found it logical she would test hitting areas that normally damaged the muscles, and finally centered on the nape which is fully protected. And it isn't too much of a reach to think that enough strikes at the same spot may damage or otherwise crack the armor so yeah. I thought the entire battle was quite well done.
Very curious to see the end of the Annie vs Mikasa showdown, and even more curious as to Annie's referring to Mikasa as a beast. There may be something well hidden behind her, such as perhaps it ends up being Mikasa's blood that's a key to this Titan chemical or something.
And lastly, the preview didn't show Eren at all but showed and spoke of Mikasa being injured. With both her and Levi out of commission, Survey Corps are gonna have a real tough time coming up.
Oh but I did have one issue with the beginning where they flashed back to that moonlight hilltop thing. This time they were all wearing Survey Corp cloaks, but when that scene actually happened it was during training just before Eren passed his 3D proficiency test.