Enabling Airplane Mode on the Lief device will make its Bluetooth radio behave opposite from normal. This means instead of its Bluetooth being enabled when on your body, it's disabled instead. Conversely, Bluetooth is activated when it's off your body, so you are only able to connect to the Lief when it's off your body.
If you're wondering how to change your Lief back to normal, you can follow these steps:
Disable Airplane Mode in the Lief App
Take the Lief device off your body
The device will make a Bluetooth Connection with the Lief App, and then immediately disconnect. The Lief App has now communicated to the Lief device that we've switched back to normal operation, with Airplane Mode disabled
Put the Lief device back on your body, and resume normal operation