The reason both of these work is because they shift your senses to focusing on something, thereby rerooting your brain in being conscious of the fact that you’re materially in the world (there’s probably a more normal way to phrase that but hopefully it makes sense).

Your brain’s trying to focus on the panic so these things give it something else to focus on, thereby interrupting your brain, if that makes more sense.

That’s why another common recommendation is popping in a Warhead; the extreme sour has the same effects as the lemon.

Another common recommendation is rubbing your hands on a textured surface so that you can focus on the texture (bit of sandpaper might work (so long as you mind how roughly you’re rubbing); bit of corduroy, maybe). That one’s doing the same thing that unknownuserunknownlocation is doing because it’s providing a texture or sensation for you to focus on.

Unfortunately, panic attacks can mimic heart attack symptoms (such as numbness or struggling to breath) but I usually focus on intaking breath. I figure, – if I can reasonably take in a full lungful of air and release it – then things can’t be as bad as my brain is trying to tell me. Deep and slow breathing, in general, is also a common recommendation.