If there is already an index partition, then we add an another partition to index server.
How the new index files are crawled to this new partition?
There is no option in content source to crawl to this particular partition.
When a new index partition is created, SharePoint will automatically use the partition. So, assuming you have one partition initially, this would contain the index data for all items indexed by the farm.
When a new partition is created, SharePoint splits the index data created between the partitions, therefore each partition will now contain part of the index data created from the items indexed.
This process takes time to complete, the Search service application is paused during index repartitioning and cannot crawl or index content. Users will not be able to run queries during this process, so this must be taken into account when planning this process.
Once the repartitioning process is complete, SharePoint will balance new index data between the partitions that the system is using.
PS - Would appreciate knowing why the answer was downvoted originally. Too brief? I got something wrong?