You need to keep your legs behind cover, and you need to shoot to slide-lock before reloading.
You only need to shoot to slide-lock if you don't want to reload with retention. Unless the COF says otherwise, you can reload anytime but if there is a round in the chamber (even if you know your mag is empty) you have to retain it. Also if your gun is empty you cannot be out in the open. You must stay behind cover to reload. If you move to the next barricade with an empty gun, it's a penalty.
IDPA is fun, but it is designed to put you in pseudo-life situations. While USPSA as you know is a 'game'. Thus IDPA's rules are much more restrictive on gear, technique, etc.