The Split option is for phases or tasks that are currently underway (scheduled to start before today and end after today), and the split will keep the current start date, but add the entered amount of weeks to the end of the phase and it will insert 0 schedule hours for any employees on the phase for the entered number of weeks. This essentially puts a phase or task on hold or pause for the entered number of weeks.
This option is available under the phase header menu.
Then enter the number of weeks to split the current phase. If the phase is not underway (scheduled to start before today and end after today), the dialog will be disabled.