I saw something like that proposed as a solution against wandering thoughts. The problem is that you can not think "hm, I really shouldn't be thinking about x",

I saw something like that proposed as a solution against wandering thoughts. The problem is that you can not think "hm, I really shouldn't be thinking about x", because that would also mean thinking about x. So when you catch yourself thinking about x, you could snap a rubber band around your arm to kill the thought.