Domestic cat suffers huge, rhythmical body twitch and it's very strange (video)

Screenshot marked up.

Well, this is on the Reddit.com website. The cat's owner asks whether his cat should be doing this. NO, is the answer but no one could say what is happening. I have never seen this before. 

Sensibly, he took his cat to the vet who diagnosed a body twitch but it is enormous and rhythmical in time with a heartbeat it looks like to me. But I am wrong it seems. It is a rhythmical muscle spasm. There you go. But don't be surprised if you find out the cat died.

Should my cat’s body be doing this?
by u/TheStockRavager in catcare

It is likely that the video will be removed from Reddit in which case it will stop working here.