This is the part that makes the framework a system rather than a toolkit of unrelated techniques. HeartMath does the physiological work — shifting the actual nervous system state, measurably, via heart rate variability. Mindfulness builds the awareness needed to notice that state shifting in real time, rather than missing it entirely. ACT then gives you a way to work with the thoughts and emotions that show up once you're regulated enough to actually look at them, rather than being swept along by them.
Used alone, each of these is genuinely useful. Used together, in that order — regulate, notice, then work with the content — they cover the gap that trips up most behaviour change attempts: the physiological piece almost nothing else addresses directly.