What ever toxic things that are in your body that come out can burn. It could be any of thousands. Toxin is any thing that the body cannot use...chemicals in processed foods, etc.
I have no idea why it is not posted to use olive oil to keep from getting butt burn. I did not know about it at first either and got burned, too. After a few live flushes I did not get any burn and did not have to use the olive oil any more.
Do not hold back a bowel movement. To do so will allow the toxins inside you that are trying to come out, be absorbed back into your system.