Xiaomi Smart Band 7 Manual Del Usuario página 25

2. To achieve the optimal performance of the heart rate sensor,
make sure the back of the band to contact with your skin.
When wearing your wristband, keep it neither too tight nor too
loose but leaving some space for the skin to be able to breathe.
Tighten the wristband before you start to exercise and properly
loosen it afterward.
Too loose
If the band can easily move
up and down the wrist, or the
heart rate sensor cannot
collect the data, try
tightening the wristband.
Just right
The band can comfortably fit
around the wrist.