In A Certain Magical Index, Accelerator says something about reversing a person's bioelectricity. What happens to a human body when he does this?
Reversing a person's bioelectricity would result in that person's death. Accelerator has demonstrated his ability to manipulate bioelectricity several times in canon. My first example is in volume 5 of the light novels of A Certain Magical Index, when he manipulates Last Order's bioelectricity to erase the virus. There are also references to reversing a person's bioelectric field.
From pages 148-149 of the Yen Press edition of volume 5 (emphasis mine):
He had so much power, but all he could think of was blowing people up by touching their skin and reversing their blood or bioelectric field-
If he could reverse the direction of somebody's blood flow and bioelectric field just by touching their skin...
"Hey, If I can control her brain's electrical impulses, I can mess with her personality data without having to use a Testament, right?"
"There's nothing I can't do. I killed people by reversing their blood flow and electric fields just by touching their skin during that experiment, yeah? I can reflect stuff already, so it's not strange that I can go beyond and control it."
Here, Accelerator confirms that he has killed people by reversing their electric fields, and implies that they exploded.
In the A Certain Scientific Accelerator manga volume 4, Accelerator demonstrates his ability to mess with someone's bioelectric field(he doesn't say anything about reversing it, however). In chapter 17, Accelerator messes with the bioelectricity of one of the members of Scavenger, Seike, knocking Seike unconscious. Accelerator's dialogue from the scene:
Did you know there's something called bioelectricity that runs inside human bodies? I'm gonna mess with it a little, heh. Enjoy!
The subsequent panels show electricity radiating from Seike's body. Blood erupts from Seike's nose and ears, saliva drips from the mouth, and the eyes roll up. The end result is Seike unconscious, with a look of anguish.
Finally, volume 3 of A Certain Magical Index references this ability. From page 189:
Those hands could alter the vector of anything they touched... For example, a simple stroke of the skin would allow him to reverse the flow of blood in [Kamijou's] veins or the direction of the bioelectric field around his body-either way, the person's heart would explode.
Here it is implied that reversing a person's bioelectricity will cause their heart to explode.