This option is a number: if greater than zero it specifies the "ratio" of the number of full crawls vs. the number of incremental crawls during an update. When set, a scheduled update will perform N incremental updates and then perform a full update. If set to -1 then a scheduled update will always try to perform an incremental update.
Incremental crawling is used to reduce the amount of network traffic required to gather documents by only downloading new or modified documents.
To perform 3 incremental updates, followed by a full update:
To never perform an incremental update:
To always perform an incremental update: