The new command created in SERVER-58270 can put the collection in a “defragmentation” mode whereby the balancer will merge and move chunks around in order to yield a number of chunks approximately equal to the TotalDataSize/DefaultChunkSize evenly distributed across the appropriate shards.

The defragmentation is a one-time operation (which potentially may take a long time) and after that the collection will leave the “defragmentation” mode.

Its progress will be able to be monitored from the return of the balancerCollectionStatus command.