To feel a heartbeat with the Bond Heart, press both sides of the device firmly until you feel the heartbeat sensation. It's important to avoid tapping during this step, as it could inadvertently wake up the Heart.
Heart will take a few seconds to start reproducing the heartbeat, so keep applying pressure on both sides until you feel it beating. Once you release the pressure, the heartbeat will stop.
Please note that it's currently not possible to remove a heartbeat from the necklace, but you can replace it with a new one through the app.