Rex had been given one of the microchips as well, attached to the base of his skull. Both Jen's and Rex's chips were wired into the network, and much more sophisticated than just controlling Jen's fake tail.
In addition to the recordings being played, the next morning, the chips started feeding in inputs from Rex. Training worked on the buddy system for a reason. The chip would be feeding input from Rex's instincts into Jen...instincts that would hopefully imprint on her.
The technology was still experimental, and there had been some side effects for the dog being networked to a human...but it had increased their success rate with the subjects immensely...and the real money was in those subjects, not the biological dogs.