User blog comment:HunterxPhantom/Chrollo's Stolen Abilities/@comment-26566783-20160727041243/@comment-25657023-20160728160247

I think it may be like a blood transfusion. The book "leeches" the Nen out of the user on the moment the four conditions for stealing the Hatsu are fulfilled. This should explain why the last condition is to touch the Bandit's Secret handprint on the cover.