Hello, This is a way to perform NAC's Captive Portal without a traditional redirect method such as PBR or Wireless redirect or even DNS Proxy. The HTTP traffic would get mirrored to NAC using a switch that supports either a policy (like the N or S series Enetrasys switch) or an ACL etc. Once the mirrored HTTP traffic reaches the NAC with the "Captive Portal HTTP Mirroring" enabeld, NAC will send back the login to the End System just like it does with PBR or wireless redirect during MAC Registration for example.
Note that your topology must be setup such that the NAC End System's traffic is mirrored to the NAC interface, therefore logically becoming an "inline" solution rather than being out of band as it normally would be. I dont know of any documentation for this but you can review the documentation in the Extrannet:
https://extranet.extremenetworks.com/downloads/Pages/NMS.aspx
Regards,
Scott Keene