opening mouth while being worked out

Question

opening mouth   hello sir, few horses open their mouth like this, showing their incisors opened up while being exercised moderately on the track in the morning. i have observed these types of horses not throwing their head up or shortening their strides or acting mean. they just open their mouth showing up their incisors and work on the track moving freely, moving on the bit or sometimes unextended. may i know why they show up their incisors likes this?

Answer
This horse may just feel comfortable holding the bit in his mouth in this manner. You may notice the horse, most likely, is without a nose band or figure eight which helps keep the mouth close to give the rider more control over the bit if a particular horse needs it.

Christopher Crocker