Simply press the button of BPM Core once to turn it on. Press and hold the button of BPM Core for 3 seconds to turn it off.
Note: If BPM Core has not been installed yet, press and hold the button to turn it on.
https://support.withings.com/hc/ja/sections/6215770022289-BPM-Core
https://support.withings.com/hc/ja/sections/360003273234-Tutorials