What exactly is neidan/internal alchemy?

[Jadespear]

...look into the book "Taoist Yoga" :

 

https://www.thriftbooks.com/w/taoist-yoga-alchemy--immortality_charles-luk/297106/item/5255171/?utm_source=google&utm_medium=cpc&utm_campaign=us_shopping_zombies_hvs_21811042479&utm_adgroup=&utm_term=&utm_content=717415192734&gad_source=1&gbraid=0AAAAADwY45hQCW-K8eSzaSBTOEWDnwrAS&gclid=CjwKCAjwwLO_BhB2EiwAx2e-39QLefe47JPnZwK0TngFVSrnRYHIhKG6vEOqRNRcMl6kNTngLwtYLRoCYpEQAvD_BwE#idiq=5255171&edition=2578262

 

All explained in there, pretty comprehensive.  

 

I've never done it, because I think it's too dangerous without knowing someone to guide you as there are MANY cases of people FAILING and Really causing harm to themselves because of improper understanding.  As noted from the ending stages - when you (the practitioner) cannot be moved for days on end and require the aid of others to sustain yourself.  

 

These kind of practices are not for lay people, nor anyone who doesn't have the time (years) of understanding and attempting them.  Please don't be convinced by anyone that the internal world is not extremely sensitive and nuanced - and is not really meant to be turned into an experiment.  There is a difference between experimentation and true mysticism/magical practices (whatever you want to call it) that deal with the unseen forces.  Experimentation is very risky, knowledgable practice is less risky.   

[NeiChuan]

My understanding is inner work/nei gong. Can argue on the details, but each school differs on what they mean. Some start other places, generally I think developing the dan tien is the best start but I've seen some great effects from other avenues. Many different ways to skin a cat.