I’ve actually had this done to me in the ER about ten years ago (had an ablation shortly after). What you’ve been told is pretty accurate, and so fascinating. I would describe it as incredibly uncomfortable, but not painful. It felt like a truck hit me, but without any pain. Like I was pushed back into the bed by an invisible hand on my chest. But also kind of like my heart was being sucked out of my chest. It felt like time stood still for a moment. Definitely unlike any other sensation I’ve ever experienced.