Without knowing how David encoded it, logic would tell me that if power is missing, eg. all my MTB rides, it would take the TIZ for HR and add to the total I have from all my power based rides.
That can be misleading, as my zone 5 power and zone 5 HR are not equal. I could be doing AC (L6) or NP (L7) work and my HR would stop at Z4. Those efforts are short and all out, but not long enough for the lag in HR.