Clarify Local-backend in tidb-lightning-backends.md (pingcap#5278) (p…

…ingcap#5280)

tidb-lightning/tidb-lightning-backends.md

@@ -16,7 +16,7 @@ TiDB Lightning supports the following [backends](/tidb-lightning/tidb-lightning-
 
 The **Importer-backend** (default): `tidb-lightning` first encodes the SQL or CSV data into KV pairs, and relies on the external `tikv-importer` program to sort these KV pairs and ingest directly into the TiKV nodes.
 
-The **Local-backend**: `tidb-lightning` first encodes data into key-value pairs, sorts and stores them in a local temporary directory, and writes these key-value pairs to each TiKV node in batches. Then, TiKV ingests these key-value pairs into the cluster. The implementation of Local-backend is the same with that of Importer-backend but does not rely on the external `tikv-importer` component.
+The **Local-backend**: `tidb-lightning` first encodes data into key-value pairs, sorts and stores them in a local temporary directory, and *upload* these key-value pairs to each TiKV node *as SST files*. Then, TiKV ingests these *SST files* into the cluster. The implementation of Local-backend is the same with that of Importer-backend but does not rely on the external `tikv-importer` component.
 
 The **TiDB-backend**: `tidb-lightning` first encodes these data into SQL `INSERT` statements, and has these statements executed directly on the TiDB node.