Sorry, I haven't been about earlier... I'm currently being pulled from pillar to post on this job - and as such haven't had time to sit down an actually look at much on any of the forums, emails, facebook groups...
The passive immobilisation part of the BECM - which is switched on/off by toggling the 'IMMOBILIZER' option in the BECM settings works as Sloth mentioned by pulsing a coil around the ignition and this then activated the fob to transmit an unlock code. From what I have noticed, this is initally triggered by the 'Key in' switch in the ignition barrel... so the BECM notices the change in state in the key switch, so knows the key is in the ignition. It will then pulse the ignition coils, get the fob to transmit a code, and disable to immobiliser - if it has enabled itself.
I call what Morat mentioned as a 'limbo' mode - where it's not in a total EKA state as the doors lock/unlock - but the immobiliser is still on - most usually because the passive part has timed out (and then usually as mentioned, it will re-mobilise itself with the key in the ignition). Entering the EKA should make it so that it will start, but it can be hit and miss if it doesn't think it's in an EKA lockout mode.
A note on entering the EKA with a Nanocom (I haven't had time yet to watch the video Morat put up) but the communication with the BECM needs to be done with the IGNITION OFF - rather than hooking up to it with the ignition is already on - the BECM communicates better with nothing on in the ignition for some reason.
Also as a side note, before I sign off again and go to get on the plane.... the passive coil (whilst it is only single direction communication to pulse the fob, it won't talk back to the BECM through these wires)... isn't just the coil, hooked up to the vehicle wiring. There is a small PCB in there, with a bunch of components too - which I don't know exactly what they do yet - I want to try and find out, as the coil on my P38, is intact I've found, but something has burnt out on the board - which is why my passive immobiliser coil isn't working properly (it wouldn't pulse the fob, and fob wouldn't transmit)..
Right, better go and get ready to board the plane...
Will hopefully have a day off soon so I can try and catch up properly!