I call this The ID ''Handshake'':
When a CA Sync creates a corresponding record for a ''new'' record seen on sync (i.e. new Defect in QC, causes sync to create a corresponding record in JIRA), it does NOT automatically report back what the new record's ID is, but it CAN!
All you need to do is setup an ID custom field in QC and one in JIRA to hold the ID of the corresponding sync'd record.
Map them as shown below. This technique really only makes sense for ID's, but you may find a use for other ''id'' fields.
Map:
***JIRA-ID (QC custom field) <-- Key (JIRA System field)*** -- This makes Sync get the resulting NEW ID and back-fill it into the field after the new record is created
***Defect ID (QC system field) --> QC-ID (JIRA custom field)*** -- This makes Sync get the resulting NEW ID and back-fill it into the field after the new record is created
It may take an additional sync-cycle for this to fill in the ID values.