Well- Im no expert on the subject, nor do I have a retention enhanced holster, but as I understand it, these holsters have a screw adjustable pin that will hook on the trigger guard of your pistol.
When a bad guy attempts to disarm you he will likely try to pull up and away from your body to relieve you of your firearm. With the proper tension on the adjustable screw, they will be pulling the whole holster with the gun. For you to retain the firearm you still need your hand on it, pressing it away from your body, allowing the pin to hook your firearm. To present the firearm, you need to tweak it in towards your body and lift.