This is going to be kind of long.

Specifically

Rule 7
Section 1 Illegal Holds/Maneuvers
Article 4.

"Pulling back as opposed to grasping the thumb or 1,2, or 3 fingers of an opponents hand"

Also

Rule 7
Section 1
Article 5.P

"bending, twisting, or forcing the head or any limb beyond it's normal limits of movement"

The 1st rule above was changed about 4 years ago where it was automatically an infraction simply to grasp any less than all four primary fingers, to what the current rule is. It was changed because the rule committee felt that in the normal course of a match it was simply too easy for a wrestler to accidentally grab less than 4 fingers and even though they were exerting no force on those fingers which could cause injury they were still being penalized.

So the way it stands now is that it is the judgement of the official (trust me I know everyone hates hearing that phrase), as to whether a wrestler has bent back 1 or more fingers in a way that would or does cause injury, and on that same line of thinking, did they bend them beyond their natural limits.

What you have to understand here is the intent of the rule which is to prevent injury. At the same time this is a contact sport, and given the nature of hand fighting from nearly any wrestling position, there is going to be some finger grasping, and some finger bending that does push the gray area. So as officials it is our job to watch this and give our best effort to make sure that it isn't a situation where an injury is likely to occur.

One other thing regarding this.

Rule 7 Section 1 Article 8.
"A wrestler applying a legal hold/maneuver shall not be penalized when the opponent turns the legal hold/maneuver into an illegal hold/maneuver"

In other words if the would be offended wrestler caused the move to become illegal by their own action it is not penalized, although the referee may need to stop the match to clear the situation. So bearing that in mind, I have seen MANY times in my career where a wrestler has a legal grasp on their opponents fingers, and the opponent moves their hand in such a way that their fingers get bent into what would be an illegal situation. However experience taught me to make sure I recognize this and either stop the hold as potentially dangerous in the case of the defensive wrestler causing it to become illegal, or penalizing the offending wrestler if they did it themselves.

Hope this helps you understand.