Use of ANT+-connected devices is fully functional in Locus Map Pro only. In Locus Map Free, it is limited to 10 minutes.
ANT+ is a wireless technology of monitoring devices mutual communication. Locus Map can communicate this way with heart rate monitors, bicycle speed/cadence sensors, stride sensors and many others. More about ANT+ and supported devices here >>
A list of tested and supported devices is available HERE >>
Switching the device connection on may need additional download of missing ANT+ library from Google Play, depending on your mobile device.