A general description for each button is below: (and/or review blog article to see example).
The 'Reverse' button will create a new timesheet that is exactly the opposite of the timesheet(s) selected. If the original entries were positive, then the new entries will be negative. So the net outcome is that the original timesheet and reversed timesheet offset each other, for a net result of 0.
Note: The 'Reverse' option will have the same effect as voiding.