I notice something like that with Pelota. Never thought of it as a head shaking thing. She seems to get in the S position and squirm in place as if to get herself comfy for the strike, then BAM its gone. She's only striked at me once though, and it was more of a feeding response thing. She smelled rodent in the air, and I was taking her out to lay down newspaper over her substrate. She seen the warmth of my face, smelled rodent, and took a quick strike. I was holding her too far away to hit me though, lol. It was too fast for me to notice the head quivering thing then. Tonight however, she was crawling all over me and flicking around my face, and I was watching her. Her head was sorta shaking a bit. Sorta like someone shakes when nervous. Never thought it was anything to do with striking, since she's such a laid back bp, lol.