The whole Heartmath thing seems to me like an exotic way to teach and practice consistent slow, regular, deep breathing (which is definitely worthwhile thing to learn!).

I believe the "coherent" HRV curve they claim brings all these benefits is simply the effect of respiratory sinus arrythmia (why your heartrate decreases when you breathe in) while breathing at a very regular interval—the HRV curve just happens to be a good indicator of how regularly you're breathing.