It isn't cracking what I do, Metal. It's simply breaking them. Let's see if I can explain it well, it is the following:
1. Take hold of a finger, it doesn't really matter which finger, though the thumb makes it a little harder, but I don't really have problems with the thumb either.
2. Push it backwards.
3. Twist it to the side, while pushing it backwards.
4. If they still don't react heavily enough on it, I twist my own arm - which I am able to do, unfortunately - therefore twisting the finger as well.
5. If step 4 didn't finish it off, you could always pull it backwards after it, though I never did that.
6. When it isn't enough after the first five steps, you could always release the finger, grabbing it again, and doing the whole five steps all over again.
I don't feel it when my fingers are pulled backwards. As a matter of a fact, I actually enjoyed it for a while. Twisting it to the side irritates me, but it doesn't hurt me too bad. But by twisting the arm of the person who has the finger will break it, no matter what happens. Especially for someone who would be stronger than me, it could come as quite a shock.
Remember that as soon as the person who've you got the finger off starts to notice the pain, you
can grab the finger with two hands to provide more strength - something I'll definitely need if my intention is breaking.
It is not as effective for other people as when I do it. That's because it can backfire, as you said. If the other person is strong enough to resist the second step, (the first step is a matter of time) then it can easily work in the opposite way - that the other person pushes your fingers instead. Unfortunately for anyone against me, I am the only person I know that doesn't feel a thing about it. I tried to make photo of it, but if I'd go further backwards with my finger, I couldn't touch the "make a photo" button
I don't have the range to make a photo of the arm-twist though, but (unfortunately) not many people can do that.
I do admit - playfully only - that someone did manage to stop me during the second step before. Though I am not sure how it would for others - as it only happened for me once - but I can easily place my elbow near the neck of the other person and still push the finger backwards. It does not give much (if any) additional strength, but it does weaken the other person by letting them lose their balance. Even if it is only for a fragment of a second, it is more than enough to get full hold of the finger.
The biggest weakness about this all is one thing: Legs. By using everything in your power to grab a finger, you're completely unprotected beneath the waist. I already found more than enough ways to delay, (or stop it completely) a kick before I get ahold of any finger, but I am not completely sure how others can stop it.
As for myself, my biggest weakness about all this is the strength in my right arm. Because I've had RSI for several years (straight), I lost a lot of my general strength in my right arm. Though my arm is recovered from any constant pain, its general strength is still quite low. Luckily, during my RSI-period, I did a lot of stuff with my left arm to prevent additional pain in my righ arm. Though I am not sure about it nowadays, I used to be able to do just about anything with my left arm as well... except writing. The way I write with my left hand/arm is horrible.
(but understandable, and better than someone I know, strangely enough)
*uhhum*
During 'playful' events and I am tired of someone trying to 'touch' me, I usually take a hold of a finger and they know when to stop. (Just about) everyone I know knows how it feels to have a finger pushed back. Now I think of it, I wonder why it didn't hurt back then. Note: It does hurt now to place the finger flat on the backside of my hand, but that wasn't always like that. I can't do a lot of stuff anymore, compared to back then.