Pickup ratio suppose to measure your chance of getting from "newest" page to the "news" page. But the more I think about it the more I'm inclined to think it's just a very hot topic detection device.

Pickup ratio > 1 means that the "newest" page has couple very interesting stories that are also seen on the "news" page.

Pickup ratio > 2 means that there is some sort of hysteria on HN about some particular topic ...