When you mine block your client sends 2 packets: when you started digging and when you finished. Due to ping problems time difference between 2 packets can differ between client and server. To make block dig rollback more rare server weakens rule, and doesn't check exact time. We can abuse this and always send 2 packets quicker.
This hack cannot make "non-instamineable" block instamineable. Instamining has strict check on the server.