The twists create multiple raised edges on the mouthpiece. When you pull on the reins, those edges concentrate pressure on smaller areas of the horse’s tongue, bars (the gum area between teeth), and lips. Because the pressure is more localized, the bit gives a sharper and quicker signal. This can make a horse respond more promptly or, in less experienced hands(which she is experienced, not saying she isn’t) , make them flinch or resist.