A bit, to a horse, is a foreign language. You have to teach the horse this new means of communication slowly and without rushing through the lessons. If the horse does not seem to undestand some components of this new language, it is time to stop, re-evaluate what is going wrong, and perhaps take a few steps backwards to reaffirm some of the basics.
For instance, if your horse will not slow down, stop or turn when you ask, there has been an error in communication and you should figure out the hole in your training rather than grab a harsher bit.
Say I wanted to learn German. If I am not understanding a concept, picking up a megaphone and yelling it at me isn't going to help. In fact, it might cause me to become upset and unwilling to learn.
Thank you, that is all.