In the snapshots, whenever you block your sword, it doesn't block all the way like in 1.7. I don't know if it was part of a new feature because nobody done or said something about it. It also happens in both survival and creative and when ever you like block and try to break something, it doesn't show like an animation, or something like that, happen In the screenshot you will see the blocking is different in the snapshots than in 1.7

To trigger it just block your sword. Also try to block hit it looks weird and different.