Fallback is automatic and transparent, protecting your data by storing a second copy of each row of a table on a different AMP in the same cluster. If an AMP fails, the system accesses the Fallback rows to meet requests. Fallback provides AMP fault tolerance at the table level. With Fallback tables, if one AMP fails, all data is still available. You can continue using Fallback tables without losing access to data. Fallback guarantees that the two copies of a row will always be on different AMPs. If either AMP fails, the alternate row is still available on the other AMP.
See Teradata Database documentation.