There has been no custom iOS IPSWs to flash since like 2009. If the IPSW firmware is not signed prior to the device being flashed your device will simply not boot. You might be correct about it being to do with NFC but the most likely thing is that they will be hooking up a wand to a computed and handling connections that way.

Maybe the wand scans all iPhones and the computer manages all the signing and I also assume wireless connections between the boxes units.

any such tool has been absolutely locked down by Apple and now even requires a VPN connection to their internal network to even work.