This policy setting allows you to specify when the peak service period begins for scheduling Exchange ActiveSync synchronization. ...

"This policy setting allows you to specify when the peak service period begins for scheduling Exchange ActiveSync synchronization. Peak is defined as the days and hours when wireless voice and data charges are highest.  If you enable this policy setting, then you can specify what time of day the peak service period begins. Specify the time in 24-hour format as HHMM. For example, specify 0800 for 8:00 am or 1930 for 7:30 pm. The user cannot change the peak start time that you specify.  If you disable or do not configure this policy setting, the user can specify the peak start time in the Windows Mobile powered device."