Sennheiser Set 860 Instruction Manual page 38

Using additional functions
38 | Set 860
If you are using several stethoset receivers and want to
protect the settings from accidental adjustment, you can
place the stethoset receivers, one after the other, into the
transmitter's charging compartment to activate the set-
tings protection function (for the hearing profile, the bal-
ance setting and stereo/mono operation).