User blog comment:HalointheSkye/Volume 3 Chapter 11 - Heroes and Monsters (IN WHICH SO MUCH SH*T GOT THROWN AT THE FAN)/@comment-25195920-20160209023127

Ok, seen a lot of people crapping on Yang getting her hand cut off and using aura to explain it, here's my theory about how it happened.

First thing to consider is that Yang has been fighting Grimm, White Fang and who knows what else for a little while at the point when she runs into Adam, she has almost certaintly burned through some of her reserves by then AND this all likely happened the same day she had her fight with Mercury (no i can't really prove that but the timing fits as does the time of day in the episodes leading up to this). Granted we do not know how quickly someone's Aura recharges but i doubt she was at 100% when this whole thing started and can gurantee she wasn't when she met Adam.

Second, Adam can put a hell of a lot of energy into his swing; we've seen this in the Black trailer when he demolished the Spider Droid. Granted, as with aura, his abilities aren't fully fleshed out but he is dangerous enough that Cinder didn't feel confident threatining him until after she had some Maiden power (yes i am guessing there but it fits). Now if his abilities work the way that has been theoriezed, he probably wasn't at full strength when he took Yang's arm but he still had enough to do the job.

Third thing to consider is Yang's aura use in that situation. From my understanding of aura (open to be corrected here) it can be used either to defend oneself or to enhance ones attacks (see volume one The Emerald Forest Part 1) with exactly what it is being used for dependent on the user to some extent. If this is accurate (no gurantee there) that means that Yang could have been putting the majority of her remaining aura into her attack, which is pratically guranteed given her personality and the rage she was feeling at the time.

Therefore, it is easy to understand why Adam went right through whatever defense Yang had to cut her arm off.

Short version: Yang was so focused on pummeling Adam she forgot to block and he took that opening.