Yeah, I've had that and still do from time to time. It's extremely difficult to describe. It's not quite like falling - it's more like the whole world stops for an instant - like an instant of utter black. The jolt is actually the reaction to it, as is the pounding heart that follows.

I'm trying to recall the name of it - most people want to call it a hypnotic jerk, but it isn't. It's something like a sleep-stops but I don't recall.

Bottom line, if it's the same thing I have, it is believed to be related to anxiety and is harmless. It's simply a reaction in your brain, but there are no true physical affects from it, other than the terror it makes you feel. And I remember when it started with me - it was BEYOND freaky. I swore it had to be a stroke or a tumor or something because it was just so weird. I totally get it.

Since I've come to know what it is, it's fine now. It will happen from time to time, but I'm able to dismiss it very quickly by simply opening my eyes. It no longer causes the dread and confusion. It's just a "thing".

I hope this helps.