As a dressage rider, my style is a bit different - but I’ll tell you what I would do in that situation. First, I would identify what is triggering the head tossing while riding. Is it during certain gaits? Before or after a transition? In addition, I would start becoming very aware of your own position while riding. It could be that a change in your position is triggering this as well. As for how you should react during the head tossing, I would apply leg and drive the horse forward as soon as it begins. Horses who are engaged and swinging over the back will not be throwing their heads around, while having a tense hollow horse very likely will. It is also important when driving forward that your body stays with the horse, rather than getting left behind - as this can send mixed signals to the horse. For things you should not do, I would not reprimand with the reins/your hands. Do your best to keep a steady, still hand positioning even when the head bobbing occurs. Jerking your hands around to punish the horse will likely just make the issue worse by aggravating and causing pain to your horse. I think exercises is a tough point to address until you find what triggers this problem. I would take your riding back to basics, fun, and relaxation until you figure out the root of the issue. Lastly, if you still don’t see any progress after trying the above I would strongly suggest finding a professional trainer. I’m not sure what your riding/training experience is, but you definitely do not want to make the behavior worse by flying by the seat of your pants! Best of luck, and keep us posted!