The ChunkSplitter is a replica set primary-only service that manages the process of auto-splitting chunks. The ChunkSplitter runs auto-split tasks asynchronously - thus, multiple chunks can undergo an auto-split concurrently, provided they don't overlap.

 
https://github.com/mongodb/mongo/tree/08651e9e7488dfb76b6fe35efdcc704209eef0f3/src/mongo/db/s#sharding-internals
 
The specification that chunks can undergo concurrent auto-splits as long as they don't overlap is confusing given that chunks are unable to overlap.