I will try to write it in a layman's terms. Caveat: this is not legal advice, and should not be relyed upon, further, everyone should do thier own due dilience.
When a corp goes bankrupt (BK), the BK law protects the assets of the corp from creditors, this protection is automatic (it is known as the automatic stay and discussed in federal law at chapter 11 of the United States Code section 362). However, the protection of the automatic stay only applies to real assets of a corp. For example, if I went BK I could put in my BK petition that I own the Brooklyn Bridge and that my creditors cannot go after the bridge, and btw, I have enough assets to satisfy their claims, the creditors of course would challenge this "fact" that I owned the bridge. In order to overcome their challenge I would have to provide some proof that I actually owned it (making a prima fascia case). Only then would the BK court allow the bridge to be protected by the automatic stay.
Applied here, WMI provided some proof to overcome the challenge by JP Morgan and FDIC that it owned the NOL. Note however, that the actual of ownership was not at issue. BUT having seen orders like this in the past, and having negotiated my fair share, I will tell you that the reservation of rights provided for JP and FDIC seems weak to me.