McFlabbergasty said:

Disclaimer: I'm not an endocrinologist, so I might have no idea what I'm talking about.

For about a decade, since my early teens, I've had an ability to induce a set of sensations within myself at will. First I feel like there is something in the back of my head that tenses just a little bit, then I feel a tickling sort of sensation throughout the backs of my legs. From the back part of the upper thighs down to the ankles. Sometimes I can't quite control the intensity. I get carried away and the tingling gets to be too much and I spasm for a second to let out the steam. I can repeatedly activate and disable this mechanism several times a second. My best guess so far is that I can somehow release small amounts of adrenaline into my system at will.

Does anybody else experience this? Any explanations?

That actually sounds more like a mild panic attack to me. At least, when I have panic attacks, they are accompanied by a weird tenseness in the back of my head, tickling/tingling in my extremities, and an overwhelming desire to spasm to release a feeling of building energy. But a panic attack will also include a feeling of dread. How do you feel, emotionally speaking, when this happens?

The mind is an incredible thing, so I wouldn't rule out the possibility that you are able to consciously alter levels of hormones, or release some neurotransmitters. There are monks that can raise their body temperature at will, so who knows?