I do that too when I realise my brain is just spewing out my mouth. It doesnt just happen when I am hypomanic (although it is more funny when I am hypomanic, I make less sense and not many people can follow my train of thought) it also happens when I am baseline but less when I am depressed (cos when Im depressed "everything I say is going to be dumb so its best to stay quiet"). Pinching my wrist definitely helps me get back in control quickly, its like a wake up call to my brain, stop what you are doing now. It works far better than anything else I have tried.