It's a nifty feature that prevents your crawl from stopping cold if you run out of monthly limit for screenshots/pages.
It can be set in (2) places:
Individual New Crawls ( per crawl )
On the Team-Account Level ( globally )
When should it be used
If you're not sure how big the site is, and you need to crawl all of it, then we highly recommend this setting.
This will crawl only up to 30% more screenshots ( of your current plan's monthly limit) - *only if you happen to go over.
Please note: Any overage will also be deducted from your next month's plan. Also once the 30% is reached the crawl will stop cold! So.. if you feel you need way more coverage, just upgrade.
Also - Even if you have selected overdraft protection, you will not be able to create a new sitemap after reaching 0 credits.