Mark X IFF triangulation and spoofing

[yahya]

I am interested about the technical aspect and factual information on the Mark X IFF triangulation and spoofing during the Cold War. The source Who Goes There: Friend or Foe? on p. 61 claims that:

Several experienced pilots reported to OTA that during the Vietnam War, they turned off their Mark XII IFF systems as soon as they entered enemy air space. Other say that more recently, on missions flown near potentially hostile forces around the Mediterranean Sea, Mark XII were turned off. The reasons are the same in both cases: a fear that enemy-or even friendly--queries would trigger the transponders and thus reveal the aircraft’s presence.
Against the more sophisticated Soviet threat, pilots expected to turn off IFF transponders long before they crossed enemy lines. If the operator believes that an IFF system increases his danger, then it will not be used to its full potential, if it is used at all. Thus, the ON/OFF switch becomes another means of denial.

Did the NVA trigger the responses from the US aircraft IFF systems and then triangulate them?