Critical strike deflect

Random Thoughts and Questions on Critical Strike Deflect (CSD). How is it calculated? Does it reduce others chance to critical hit you. Say you have 5%, and they have 20% crit. Do they functionally have 15% crit against you? OR if they roll a crit, do you have a 5% chance to deflect it, making them have functionally 19% to crit, and 1% more to 100% deflected.