If you have 3 sequential operations 10, 20 and 30 on a shop order. Then include operation 20 in an operation block. Then you schedule this operation block so that operation 20 ends up on a later date/time. This will always force operation 30 to be reschedule since it can start only once operation 20 is finished, but what to do with operation 10? If you leave the Compress Operations check box cleared, operation 10 will remain as originally scheduled creation a slack between 10 and 20. If you select Compress Operations, also operation 10 will be rescheduled so it should finish just when operation 20 should start.

If operation 20 instead is rescheduled to an earlier time, this instead controls whether operation 30 should remain as originally scheduled or be rescheduled as well instead. 

I hope this helps! 

@Björn Hultgren thanks for the explanation, great help!