Try to keep your hand motionlessly. You got it? Actually it is impossible, your hand always moves.

These fluctuations contain some internal sources like breath, heartbeats, muscles movements and external sources like wind, shivering car or whatever else.

 If your heartbeats are strong enough it is quite easy to define your heartbeats rate, just collect hand fluctuations using built-in iPhone accelerometer, spread out it into flat spectrum and find the highest spectrum line.

 Spectrum with strong heartbeats:

 But if your heartbeats is not so strong, or you keep iPhone in heavy or soft case, or you got strong noise or shivering, the task becomes more difficult. 

Spectrum with weak heartbeats and strong noise:

 You need to use filter to cut off noise out of usual range of human heartbeats rate.

 Noise filtering function:

Actually this function is already implemented, you just need to set values for low and high thresholds in settings section of iHeart to customize your personal heartbeats range.

Filtered spectrum with strong heartbeats:

  Filtered spectrum with weak heartbeats and strong noise:

 But even your heartbeats are weak or you are at shivering place or your iPhone is inside of heavy or soft case, you always can help your iPhone to calculate heartbeats rate confidently enough by moving it in time of your heartbeats.