On your SETTINGS>Global Employee Permissions page if you let employees trade shifts or drop/pickup shifts "At Will", they can simply trade shifts without manager approval. Employees can post a shift to the tradeboard and if someone agrees to trade or pick it up, then the schedule is automatically updated without manager involvement.
If you allow pick up of Unassigned shifts "At Will", employees can pick up shifts up to their set maximum hours per week - any shift putting them over their maximum hours per week will be sent to the manager for approval.
SETTINGS>Global Employee Permissions page).
Example: John wants to drop a shift. He clicks on his shift and chooses to place it on the tradeboard as a "drop". Jane sees his shift on the tradeboard and clicks on the shift and agrees to pick up his shift. If you allow drops "At Will", the schedule would then reflect Jane as the worker for that shift. If you allow drops only with "Manager Approval", then the shift is removed from the tradeboard and a pending trade request notification is sent to the manager and is also displayed in the "What's New" section of the Manager Home Page.
To track who picked up or dropped each shift, click the shift in any schedule view and then click the Shift History link in the shift change window.