Heng Ha breathing, in the bajiquan I learned, was about power release determined by the type of strike used and direction of the strike. It can be trained in the neigong part (also reverse breathing) but also in the da qiang exercise, especially the dou qiang one. I've known people who haven't trained in it and their punches are extremely powerful & effective - hard to say how critical it is to the systems (below is a line although it is not from my line of bajiquan):
http://www.wufamilybajiquan.com/technical-features.html
Internal training: As indicated above, BajiQuan makes use of various internal exercises in order to improve the control of Qi. These exercises can be used either for martial applications (generation of the Explosive Power) or for therapeutic purposes. BajiQuan uses classical respiratory exercises (natural and contrary abdominal breathing), but it focuses more especially on an explosive technique of Qi circulation (called Xing Qi), which is based on the use of the “Heng” and “Ha” sounds punctuating the power releases.