From whence flows the legendary supernatural 'power' (or abilities) of the Mystic, the Taoist sage, the alchemical Adept, etc? Many have what are more properly termed 'opinions' than theories on the subject matter. Working from my personal somewhat extensive experience in this field, not by way of any sort of "book learning'". I will herein share a factual, in depth exploration of the topic.
First of all, every high-level Adept will inform you of the same Truth, namely that the Source of their 'power' is within, yet also not exactly what they would call 'under their command', that is, under the control of their egoic self/persona. This is in large part due to the self-preservation instinct of one in their position. There are certain things the wise no longer test. Rather, one working with this supernatural power or transcendent abilities realizes just how much one's body is primarily a conduit, or 'channel' (such a loaded term) of a Greater/Higher energy. While (for those of whom I speak) this Greater/Higher energy is that of their True Self, the body must still be developed to handle this energy safely, in a gradual manner, so as not to unduly burden the physical/emotional/mental structures of said body... in brief, one must train to avoid burnout. This sort of burnout can be deadly. In fact, the power over life & death is in the hands of all Adepts of a certain level of 'attainment'.
So how exactly does an Adept come to have this sort of supernatural ability? I'll tell you. Our Adept has taken Nature (the Divine/the Universe/God/Goddess/Etc) as his/her bride (or groom, I suppose, but let us not stretch historical illusions too far beyond their customary bounds, at least for today). One who is madly in love will do anything for his/her lover, and so it is with the Adept, who has developed a habit of following his/her inner impulses to satisfy Nature's desires. Nature, in turn, responds in kind, and it is at this point that the fun really begins!
The 'Secrets of Manifestation', becoming so nearly blasé in our day, are merely the point-of-departure for our Mystical Adept(s). Rather than manifesting to serve one's personal desires, or even to satisfy the desires/demands of other beings (human or otherwise), our Adept has become led to apply his/her whole life-force to manifest and support the courses established by Nature Herself. The Adept attains Mastery only after fully submitting to the Higher Mastery of Nature. As a dedicated agent of Her Will, the Adept consummates this 'sacred marriage' (Hieros Gamos) by way of an undying (note my word choice here... no coincidences!) obeisance to his/her Beloved. The Beloved then begins Her pursuit of the Adept in earnest, now waiting on Her lover, granting his/her wishes (literally), often before they are realized. She begins planting the seeds of desire for the gifts She is about to bestow. This is a relationship of purest passion & total devotion. Go read some Rumi or any mystical poet.. mystical poet being something of a redundant phrase, for True Poetry is the (at times unrealized) result of the writer's pursuit & love of Nature. Fall in love with the Divine, and you too will experience all these things!