There is little in the world more difficult than trying to communicate with a guy in the middle of a blow job you're performing. Unlike cunnilingus, when there's nothing (or very little) actually in a person's mouth, giving a (good) blow job requires that you put a dick in your mouth, and that kinda makes it difficult to talk, not that you should be doing a lot of talking (though, humming, I've been told, is good). On the other hand, one does run across the occasional dude who wants to make it clear that he wants to ejaculate in your mouth — whether you intend for him to or not — by either holding onto the back of your head or thrusting into your mouth. And, sometimes, your bodily functions are going to trump his. So, what is the universal sign for "Get your dick out of my mouth, asshole, before I puke on it"?
Look, I'll admit, this has happened to me exactly twice. The first time was in college: I was on my back, he was fucking my face and I felt that tightening up in my jaw that signals an imminent attack of TMJ-induced lockjaw. Unfortunately for me, he was a little too into what he was doing to notice I was politely trying to convince him to quit thrusting; unfortunately for him, my jaw doesn't exactly lock all the way open; but fortunately for both of us, he wasn't particularly well-endowed. He stopped pretty quickly after that. Word to the wise: if, perchance, you happen to catch a little foreskin in your back teeth in the midst of an oral-sex induced episode of jaw locking, the appropriate reactions to a man's bleeding dick are not: a) hysterical laughter; b) saying anything like "Well, it serves you right."
The second time, I was in my mid-twenties, he was much larger and more experienced (and 45), but he was no less disinclined to notice me signaling that things were not going well with him trying to fuck my face. This time, it wasn't my TMJ that got the better (or worse) of us, it was my hypersensitive gag reflex. Nothing kills a mood like vomming down a guy's dick.
So, what is the sign, short of grabbing his nuts in an unpleasant fashion or slapping his torso? Or should men just consider it a life lesson in what happens when you are less than appropriately appreciative and thoughtful while receiving one?