Alright, I have two cats in the house, one big, fat male cat and one half the size of the other, fit sized female cat. Whenever the BIG one lies down somewhere, he’ll twitch only when he is touched by something. Why is this? Is it because he is ticklish (if this is even a word) or something else?
Note: The big cat hisses, slashes at, and always run from the smaller cat.
Yep, my guess would be ticklish. Want to bug your cat? Wait until he is asleep, then very, very lightly barely touch the tips of the hairs in his/her ears, or the tips of the whiskers. These are very, very sensitive, and if you do it right you’ll annoy the piss out of them until they finally wake up completely and bite your hand off.
Of course, cats will do some weird things when they are dreaming. I’ve had several cats that chased something or other - their legs and whiskers would twitch like crazy. Investigating peculiar sucking noises revealed several cats that reverted to kittenhood in their dreams, and were ‘nursing’ in their sleep. And one of my cats must have really strange dreams - while sound asleep, his tail will slowly fuzz up into the typical bottlebrush, and sometimes he will jerk it back and forth. Must be fighting monsters or something.
You’ve got a point there, brother - and one of these days I’m going to figure out what it is. - Cecil Adams
I’ve seen my cat do this too! Personally i think it funny as hell to watch. twitch twitch 
I think they do it when they itch actually. The reason is because i have pet my cat before and run my fingers through a tangle and it pulled his hair a little bit. Sometimes this makes that one little spot on his back twitch. I’ve also noticed him do this on occasion when he’s lying about (which is most of the time, i have a lazy kitty :)). So, it’s an itch, that’s my theory…
“Through twilight, darkness and moonrise
My scarlet tears will run
As stolen blood and whispered love
Of fantasies undone”
Thanks for the help! I also guess that my cat is so bothered being tickled that he actually meows at me whenever I touch the chest or the belly part of him.