This stuff is pretty much over my head, but if I understand correctly, the fact that the pilot’s feet (and body weight) aligned with the main thrusting mechanism (right under their feet) created a kind of advantageous situation not unlike a Segway, perhaps. Evidently it could make quick adjustments to its thrust patterns to prevent tipping, and in fact (again, like the Segway) actually used the pilots bodily leanings to steer in the desired direction.

Of course, most important of all is that it was nicknamed the Flying Pulpit!
https://www.jetsprops.com/prototype/sky-high-dreams-and-grounded-realities-the-tale-of-the-x-jet.html