The generic extraction works by storing the last time an init or delta was done. For example - if you inited the extractor today - then once the init is successful - SAP ERP will store the init timestamp in its tables - I think it is ROOSGENDLM but am not too sure.
The subsequent successful delta timestamp is also stored and when you extract the next time - ERP will then look at the last delta timestamp and then determine the changed records. Once the data load is successful in BW - it writes back to ERP for the new timestamp.