NOTICE: It may not need saying - but this is all from my experience, not necessarily fact and certainly not the ultimate word.
Why don't you understand what "Internal" means yet?
It's because you don't understand what Chinese term is referencing. If someone said Internal - ask them to clarify.
For instance - in this very forum the section is called "Nei Jia" and references "The Internal Martial Arts".
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First, we need to look at the terms we are so often [mis]using!
Nei Jia （内家）is understood as both a geographic and family-transmission related term. Wai Jia (外家 ) means outer family. These are often translated simply as "Internal."
Better translated - they are "Inner/Within Family" and "Outer/Outside Family".
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Okay- but then what does internal practice mean?
Internal Practice/Skill in Chinese is called Nei Gong (内功). While it has varying functional definitions - it is the term most are referring to when they say that an art needs the "Internal" component. In very vague and general terms these are often practices that place higher emphasis on postural/structural, breathing, and mind coordinations.
External Practice/Skill in Chinese is called Wai Gong (外功）. Generally this refers to practices that place the most emphasis on the physical movements. E.g. ) "Do This", Now Step and Punch, Now Back up and Block, etc.
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Okay, so what is it!.
It exists as practice methods - the reason you may not think it exists is because you have not been exposed to any "Nei Gong" practice methods. Now that you know the terminology - you can start your own google search for Nei Gong, or the chinese terms 内功。
Another reason you may not believe is that there are semi-inconsistent definitions of it. It is a general term for Practice methods that are commonly associated with healing arts in chinese and is not specific to one or two methods.
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What's the difference then between internal and external?
In my experience:
Most practices are not mutually exclusive (purely external or internal), but there are good definable examples depending on who you ask and what level is being taught at.
Internal Practices often include very detailed breathing mechanics (that include physical elaborations on how to breath - speed, rate, sensations that assist), mental elaborations (that may include specific actions, parts of the body, or abstractions to focus on, or other), and after those are understood - sometimes a coordination of those combined with a physical elaboration (may be detailed postural work, structure refinement for tension reduction, static and dynamic postures, transitions, or other).
In some schools of Tai Chi, the Nei Gong is taught as a light contraction in specific parts of the abdomen during certain postural transitions and postures.
In the Shaolin ChanWuYi school, there are a wide variety of practices called Nei Yang Gong (内养功）- which means Internal Building Exercises. Similarly, there are Internal practice principles/guidelines (or Lian Fa 练法）that assist in incorporating these internal practices into every day movements - including coordinating specific mental coordination, breathing, and postural adjustments that can vary. (You can learn some basic ones on Chinese youtube directly from Shi De Jian's disciple Chan Sui Yin who teaches out of Hong Kong and has a youtube channel)
These are also related to the "Ba 把" from the shaolin phrase "Practicing Martial Arts without practicing "ba" is akin to swinging wildly (literally, recklessly hitting). It's preferred to teach 10 styles of fighting than teach a single "Ba" [implication that it is due to the responsibility of understanding something as powerful as such a "Ba"].
”练拳不练把 等于瞎胡打， 宁教十趟拳 不教一个把“
There are many forms of laying, seated, standing, and postural nei gongs as well and many styles have their own incorporated practices that are often not taught to beginner students for some reason. I believe there are even books in english available on Nei Gong - although I cannot attest to their quality.
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So why can't people explain why these Internal Practices are referred to as so great?
At a certain level, these practices cannot be described in words. Some arts have ways to check them (such as in Tai Chi there are pushing hands/sensitivity methods to test postural alignment), some depend on body feedback from various feelings, and may even be quantifiable if you could isolate the exercises (nei gong) to measurable things such as frequency/size of your poop, blood pressure levels, and more - all the way up to less quantifiable things such as subjective mood levels, abilities at work, sleep quality, etc.
Since the basics lay a path/foundation - the higher levels generally cannot be explained and are something that must be gained through experience on the path. The best way I could think to explain is that the Buddha attained enlightenment. He could not give you enlightenment even if he showed up in front of you - it is a path you must walk.
You could say the same of external skill though - that the body simply needs to be adjusted to it and over time through practice will be better.
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So where's the magic?
Don't know - I think people practicing looking for magic powers may end up disappointed. People looking to improve their health and general life abilities may be surprised if they can find a good teacher.
The magic IME is that most Nei Gong I have seen serves to relax, and naturally allow elongation and release of the Spinal tissue (Spinal Fascia tissue? I'm not an anatomy expert) while facilitating better blood/oxygen flow to the central "Dan Tian" region of the body.
Those improves comfort in lower back, reduces pressure along spine, and serves to assist in realignment/weight distribution through the pelvis. Those, IME, are the very foundational level benefits and the secondary, tertiary, etc. benefits are the "prime" reason for many people's practice - re: spiritual understanding
Summary "TL; DR"
NeiJia / Nei Jia Quan is not what most people mean when they are talking about "Internal" skills of someone.
They are talking about Nei Gong - which means "Internal Practice/Skill" and has practice methods that exist and can be easily found online or from a legitimate teacher now that you know what term you are searching for.
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Originally Posted by Dragonzbane76
Internal(neijia) is just the name of a category in TCMA that emphasizes on more refined body mechanics as in spiral movements, 含胸塌腰（or 拔背）the unity of the body in motion, .... There are many similarities between the internal arts and the external arts but most people who have trained them both agree that arts like taiji,xinyi and bagua share more similarities regarding what I mentioned above and can be categorized in the same group. 
