A lift stun is a stun that causes the opponent's chin to jump up, hence the alternative moniker "uppercut stun," as it looks like they were hit with an uppercut. Lift stuns are useful because they typically last 60 frames and are some of the deepest stuns in the game (you usually don't get a full +60 from them, however, since you have to factor in the recovery frames of the move you used to initiate it). Nevertheless, they provide over 23 frames of stun even if the opponent executes the fastest possible stagger escape, and are thus common lead-ins to critical bursts.
If you tell me some characters you use, I can give you a few examples of attacks they use that cause lift stuns.
I'm not sure if you're familiar with Hayabusa, but a few of his are 6K, 6P+K, 8P (NH), 3P (CH), & ongyoin 6P. You can see Hayabusa's 6K cause a lift stun at 0:48 in
this video, as well as at 0:58.
I'd actually love to hear 'em. I remember ones like KK, but if I forgot any, it'd be nice to know.
Oh, and I doubt you'd know the frames, but if you do know how long any of the sit-down stuns last, I'd love that info, too.