Normal SELECT does not see deleted rows
1	1	1
3	3	1
With the setting disabled the deleted rows are visible
0	0	0
1	1	1
2	2	0
3	3	1
4	4	0
With the setting disabled the deleted rows are visible but still can be filterd out
1	1
3	3
Read the data after OPTIMIZE, all deleted rwos should be physically removed now
1	1	1
3	3	1
5	5	1