I don't pretend to know the details of how Boeing and others get these tailless designs to work, but when I was lucky enough to have some contact with Mark Sumich, program director for the X-36 project, I found that they had successfully created a YAW vectoring exhaust nozzle, as well as some pretty sophisticated software to control it. Part of that system included a rate gyro for the yaw axis, connected to a vane on the outside of the aircraft, which served as the "trigger". Maybe these new designs are an evolution?