This is what Rebbe Nachman tells us about Pidyon Nefesh in Lesson 3 of the Second part of Likutey Moharan:
"The only good time to heal the ill by way of remedies is specifically after pidyon and sweetening, for only then, after pidyon and sweetening, is a doctor empowered to heal."
We'll see what he's actually telling us in just a moment. But before we do, let me quote you something else from Sefer HaMiddot, (The Book of Traits), where Rebbe Nachman explains the spiritual process that occurs when a person gets ill (the italics are mine):
"When suffering (ie, illness) is sent to a person, the suffering (ie, illness) is sworn not to commence until a specific day, and will not leave until a specific day and hour, and through the using of a specific drug. However, repentance, prayer and charity annul the vow."
In practise, this means that spiritually, it would be very difficult for anyone to truly get healed - and if you look around, we actually see that this is what's going on. Yes, there's a lot of 'temporary' healing, and partial cures, and periods of respite, but especially when it comes to chronic illnesses, many people continue to suffer for years, and years, despite all their efforts to get well again, and despite all the 'experts' they go to for help.
But the situation can change drastically after someone does a pidyon nefesh.
A pidyon nefesh is a sum of money that we give to a genuine Tzaddik (holy person), to pray for the judgements that are on our soul in the spiritual realm to be sweetened and removed. The bigger the problem or health issue, the bigger the amount of money we should give for a pidyon nefesh.
Once the judgements have been cleaned off the sick person's soul via a pidyon nefesh, THEN ANY DOCTOR CAN HEAL THEM BY ANY MEANS. The practical implications of this are enormous.
It means that you go to doctor X who tells you medicine Y will heal you - and it will! Or, you go to a naturopath W who tells you eating 5 kilo of seaweed will heal you - and it will!
Or, you go to a Su Jok person who stabs you in the hand with their metal pointer, and then sticks a lentil or two on your sore spot - and it'll heal you! And so on, and so forth.
The real healing already happened in the spiritual realm, and now that the harsh judgement has been dissolved via the pidyon nefesh, it takes very little 'hishtadlut', or practical effort, to regain our health.
There's a few things you should know before you rush off to do a pidyon nefesh:
1) You actually have to believe that it will work, for it to work (not least, because otherwise you'd never actually pay the money out in the first place.)
2) You need to give the money to a bona fide tzaddik, which is not always so easy. Two holy rabbis that do pidyon nefesh properly, and who I have used myself, are Rav Shalom Arush and Rav Ofer Erez. (Get in touch if you want details for Rav Ofer, and I'll try to find a phone number.)
3) It doesn't necessarily have to be a lot of money to work, although clearly the bigger the problem, the bigger the amount required for a redemption to be successful, as big problems and serious illnesses mean big spiritual judgements.
For example: I know of one case first hand where a baby with a heart condition was given less than a week to live. Friends of the family donated 180nis to Rav Arush for a pidyon nefesh (around $45) - and two years' later, that kid is still alive.