How a 100# woman can take out a 300# man with one finger.
Really and truly.
First, you need short nails. Sorry.
Then take a hand towel, fold it in half and in half again, and put it on a solid table, sit down and make a hand like this, and tap, not strike, downwards against the towel covered surface.
It's very important to feel and know what is going on with your hand. When you tap downwards, let the other fingers cradle and give support to the middle finger while keeping the basic form of the hand.
Toss the pain/gain theory out the window. Tap like you are tapping a drum. You are doing nothing more than telling your nerves "be prepared for more of the same" and your hand bones "new stress at unusual angles will be required", and that's all. If you are in touch with what you are feeling, at some point you will decide you've had enough, and you will be right.
Take at least two days off, no exceptions. Make a point of keeping calcium in your system. Your fingers feeling "weird" is OK, pain is not. Nerves sometimes send memos to your brain when you're doing something they're not used to.
After a week or so you'll be feeling more comfortable with it, so you have to be twice as careful. Be mindful as you start using both hands, like a drum solo. Start out light, increasing force in increments, while feeling what is going on, so damn important. You'll feel when you've reinforced the message to your bones, "add more calcium".
The next stage, the weirdness goes away, you become comfortable with your drum solos and popping away a mile a minute. Now be twice as careful again.
Stand in front of the table with your feet apart like you're straddling a horse, and after warming up with your drum solo, start "sticking" the towel like you're stabbing down and holding it down. Feel. Assess. Go back to drumming, then "stick" with the other hand. Gradually and carefully, put a little hip motion into it, turning your hips as if your body turning drives the arm that "sticks" the finger...but add NO additional force whatsoever. Just feel the motion, smooth, graceful, like you've suddenly turned on the water, and the decision to stick flows from hip to shoulder and delivers down the arm.
Two days rest minimum and calcium as usual, then put the double folded towel on a vertical surface with tape or thumbtacks. Still do it downwards, but also do it from different angles with the towel tacked up. Drum rolls, force increasing by increments, before "sticking".
In a couple months you will feel very comfortable sticking very deep, and you should be cautious to the point of obsession because injury is just waiting for you to get overconfident. You can start practicing little scenarios now. Sudden strikes, strikes from strange angles, from confined space, as part of a subterfuge, etc. Let the "roll" from your hips increase the force, always by the barest increases of power, and study the results. And you can try folding the towel only once...FEEL FEEL FEEL. If you're not ready, you're not ready. If your body is healthy, it always comes.
When you are ready, go single ply on the towel. Now you're not protecting your bones as much as making it comfortable for the tips of your fingers. When you're ready to graduate the basic class, ditch the towel.
Intermediate class. You are now a pest control service, lol. In addition to drum rolls and "sticking" once a week, now, every six legged critter from a roach running to a fly landing is a target for your middle finger. Yes, do it when no one's around so you don't look like a loon. Focus, and feel.
Not only (after some funny incidents) are you going to start getting very accurate, very quickly, your whole body will come to attention when you see a bug, that you see in a cat when it registers random movement. This is A Good Thing.
Advanced class: add to your crimes against the insect world, the abuse of fruit. Make a fingerhole in a cantelope. That was easy. Make finger holes in a watermelon...well, that takes a little bit of focus. Now try to take a divot out of an apple. Figure that out, and you can try light veneer wood like they use for paneling. You will not need to strengthen your fingers past this point, but the lessons in form, "snap" and penetration are invaluable.
Your hands will not be deformed and your fingers will not be callused, if you were careful your career as a concert pianist will be unaffected.
Post graduate. Study anatomy. Put your hand on your windpipe and feel what the tiniest amount of force against the prominent cartilage forward does. A "snapping" shot here takes very little force to send a mammoth on steroids puking and gasping while you run for it. Now feel further down your windpipe until it disappears behind your breastbone, a little notch there where your fingers settle very naturally. If you can't get away, and plan to "stick" with power, your fingertip might roll off the windpipe elsewhere, here it sinks right on target and delivers all your force. He will have trouble breathing temporarily, and if you flatten the gristly cartilage that keeps his windpipe open, perhaps permanently. Last extreme...the thin bone behind the eyeball has a suture in it that makes it easier for a "stick" to pierce to the brain than piercing the wood veneer. And even if you never chose to take your conditioning that far, eyes are very vulnerable. And just a thought, because extended fingers have that much more reach, his eyeballs may be in reach of your fingertips before you are in reach of his grasp.
If you can hit a roach on the run without looking (and this is much easier than it sounds, given time), these body targets are as easy to "tap" as party balloons before you attempted this crazy regime.
Wise advice from the master: float like a butterfly, sting like a bee ;)