Check this if you want the instance to be stopped instead of terminated when it times out due to being idle. This allows existing instances to be used when provisioning a slave. You would use this option to preserve the state of the instance, including that of the EBS volume connections.
Note that instance store content is lost, as are running processess and the contents of memory.
Stopping and starting an instance starts a new hourly billing cycle, so short stop timeouts are counterproductive.