I found this: https://www.snaq.ai/blog/cooled-rice-blood-sugar-research

My guess is that cooling the rice causes some of the starches to polymerize into longer chains that are harder for your body to digest, which lowers the glycemic index of the food.

I also prefer very short grained rice, which has slightly higher protein to starch ratio and reheats better. This stuff is often sold as "sushi rice." If you're on a budget, Calrose medium grain is fine. (If you're not on a budget, tamanishiki koshihikari is my favorite bougie short grain rice.)

Afaik it's literally cook rice then cool it in fridge overnight before you use it. Something about the process makes the starch change and it's better for people with the beetus. I am not a food scientist and idk how it works.