The sage advises the queen not to spin the cylinder. Since the king just fired an empty chamber, we know the cylinder was set to 1 of the 4 empty chambers, otherwise he would have shot himself. If it was set to the last empty chamber before the 2 bullets, then the gun now has a bullet in the firing chamber. If it was set to any of the other 3 empty chambers, then the queen should be safe now. She has a 3/4 chance that there is not a bullet in front of the hammer now.

If she spins the chamber, 2 of the chambers are still loaded, and 4 out of the 6 are unloaded. So if she spins it, she has a 4/6 chance of no bullet (or a 2/3 chance of living). 3/4 is greater than 2/3, and she obviously wants to live, so she simply pulls the trigger again.