I have been able to induce at least some form of pseudo sleep-paralysis basically at will. This was learned, quite unintentionally, while meditating. I'll call it, for lack of a better phrase, wakeful paralysis. Wakeful paralysis is to sleep paralysis as Lucid dreaming is to Vivid Dreaming.
I've no idea how to transmit this experience, but perhaps it can be said that there are forms of physical meditation, which may lead to 'tricking' the body into paralysis.
The Dangers of Wakeful Paralysis ∞
Wakeful paralysis can be the most frightening experience a person can conceive of. Some sensations may include fearing drowning in one's saliva, being crushed by the weight of one's ribs, or being loomed over by dark apparitions. These are not "silly little fears" but as absolutely real as your literacy is to you now. The mind plays very real tricks, especially when one's senses follow along.
Realistically, as sleep paralysis is an expected and usual thing for the body to undergo, there are probably few side effects to the wakeful variety.
However, the delibitating mental anguish which wakeful paralysis can bring upon one could present long-term concerns after the immediate effects have worn off.
Furthermore, experiencing such paralysis more often than the one planned evening's worth, as with wakeful paralysis could be a strain on the body's resources in subtle and potentially dangerous ways. The feelings of apprehension alone could pose serious risks on blood pressure or the heart.
Heck, at the very least.. one is burning off a lot of Vitamin C being stressed.
Breaking Wakeful Paralysis ∞
Again, as i'm only familiar with my own brand of paralysis, this may not apply to others or their circumstances, mindsets or physiology.
I have found that the best way to break paralysis is to accept and stop fearing it. As I understand, the hormones in the muscles work that much harder when there are attempts at mobility. Relaxation, especially calming one's mind (close your eyes and focus on your breathing) is essential.
There is a crouched-tiger type of apprehensive feeling in you which must be kept at bay until you feel the cage of paralysis weaken. Then let loose! I normally keep a dual-mindset and calm myself until my anger can be effective. I usually 'wake' out of paralysis like one would wake from hearing a sharp noise in the night. This is generally followed by a heartrate that would make your grandfather keel over. Careful.
The Benefits of Wakeful Paralysis ∞
Ok, so just why in the hell would anyone want to undergo something like this? Well.. imagine being forced to face your worst fears. The experience of wakeful paralysis completely overshadows a vivid-dream nightmare. It's real, you're awake and completely helpless!
In a sense, it is an interesting challenge. Not only can one meditate into a state to throw one's body into a semi-sleep comatose state, but one has the opportunity to test one's coping mechanisms.
I myself had used wakeful paralysis to more closely examine dreaming and comatose states. Now I risk sleep paralysis in meditation to calm my body and examine the border between wakefulness and what is best described as a lucid-dreaming "snippit" at the beginning of a form of sleep. I review the images and mental states I slip through as I meditate closer to wakeful paralysis.
Risking death for fun and profit.. and all that. =)