I was curious about this too
I’m just keeping an eye on it for now as it’s somewhat trivial for small changes.
HR spikes above 185 bpm fixed
Heartrate spikes are detected and fixed by comparing the HR trace to your max HR. Spikes above max HR are replaced with a line interpolated between the HR values before and after the spike. If the HR data for an activity is very poor you can ignore the HR trace by clicking the cog icon. You can see the unmodified HR trace by clicking the charts button and adding Raw HR.
So anything over your Max is just capped as that, nothing more technical than that to my knowledge.