Hey there 3CPO (or what was the name again? 
)
As SAP HANA comes with two storage engines for table data, there exist two different ways to keep information required for an UNDO.
For the row store tables, the old data is internally saved in virtual UNDO files and removed after the transaction that created it finished.
For column store tables all data changes are happening in the delta store anyhow. As the delta store is an append-only data structure the old version of records (that's what is required for the UNDO) are present in the main, the delta or in both stores. To perform the UNDO all that's required is to mark those records as 'current' again.
- Lars