I have been able to do this for years and out of my curiosity i wanted to research it but yeilded no results; so i thought i should ask about it.
It's like i can tense all my muscles at once and send a "shock" through my body causing it to shake, as well as increase my heartbeat.
From what i can tell it begins when i sit or stand still and focus on tensing muscles in the back of my thighs and calves, from here it moves to my diaphragm, ramping in intensity across my body, ramping up, causing me to lightly shake until the shakes become great enough to make me lose my physical concentration. Afterwards the heavy beat of my heart is very apparent as well as my awareness of the lack of breathing through this "event".
Can anyone, for the love of god, tell me what it is (the cause) and is it safe?
I have been able to do the same thing for a few years. I also have needle phobia (caused by vasovagal shock) and the feeling I get when I tense my muscles like that feels a lot like the feeling I get after before and after being injected. My needle phobia temporarily lowers my blood pressure and leaves me looking anemic - could this be related?