This is a drill saw for the first time from my head coach Scott Guttery when I arrived at Fort Myers High School. This is a great drill for all of your players, not just your post players. In my opinion, I think this is actually a better drill for guards to learn how to quickly get a shot off around the rim.
Really emphasize quickness and explosiveness throughout the entire drill. Also make sure that your players mix in both right and left hand shots.
For older/advanced players, have a coach/teammate create some contact through the drill to make it a more uncomfortable situation.