AT&T SYSTEM 25 Manual page 168

Splitting One-Way Automatic
Attendant Splitting One-Way Automatic is a standard step in call extending procedures.
It allows the attendant to converse privately with a called party while the calling party
is on hold. Splitting occurs when the attendant presses START or a Selector Console
button. Music-on-Hold, if available, is not provided to the split-away party.
For more information, see the entries for "Extending Calls" on pages 3-17 and 3-21.
3-51
Splitting One-Way Automatic