How does a Read on Write snapshot handle modifications/deletions?

A read on write snapshot will cause any changes/updates to be redirected to new blocks. It's easy to see how this can work if data is to be appended, but what if data in the block is modified or deleted? Since the snapshotted block can't be modified, how is the information of what is modified or deleted applied? It can't be just metadata from here on out, right? That would really slow things down if the data is to be used for analysis.