The actions of backflushing when you close an MO depend on the closing option you select. If you perform MO receipts for only 75% of the planned items, then only 75% of the items marked to backflush are backflushed. However, some processes require that all 100% of the materials be issued and the remaining 25% are not recoverable. Thus, if the correct closing option is taken, the remaining 25% will be backflushed as the MO is closed.
There is a significant amount of detail behind the several closing options. We detail all of these in our book Shop Operations. You might want to get a copy from our web site.