According to Brad Zdanivsky, a 36-year-old Canadian quadriplegic climber, as reported in this article in BBC:
Some people do that in sports by clipping off a catheter to let the bladder fill - that's the easiest and the most common - and you can quickly get rid of that pain stimulus by letting the urine drain out. I took it a notch further by using an electrical stimulus on my leg, my toe and even my testicles.
It is called "boosting" and it takes advantage of something called "autonomic dysreflexia" (AD) to increase blood pressure and performance. According to Wikipedia " AD occurs most often in spinal cord injury individuals with spinal lesions above the T6 spinal cord level; although, it has been known to occur in patients with a lesion as low as T10. But in addition to the increase in blood pressure which is the point of "boosting" AD is also associated with throbbing headaches, profuse sweating, nasal stuffiness, flushing of the skin above the level of the lesion, bradycardia, apprehension and anxiety, which is sometimes accompanied by cognitive impairment. Further Zdanivsky says "you are getting a blood pressure spike that could quite easily blow a vessel behind your eye or cause a stroke in your brain," and "It can actually stop your heart. It's very unpleasant, but the results are hard to deny."
Enjoy the Paralympics, they start TODAY!