doppel doesn’t directly change your heart rate. It works by changing your brain’s perception of your heart rate.
A useful analogy is to compare it to music. Upbeat music gets us going, slower music is calming. This isn’t a coincidence, we actually entrain to these rhythms - a faster rhythm makes us more alert, and a slower one calms us down. doppel is doing a similar thing but with a beat that you feel rather than one that you hear. Like with music, your heart rate won’t match doppel’s rhythm however if you have a heart condition we do recommend speaking to your doctor first, just in case.