As you release the trigger, most pistols have a "click" you can feel when you have released the trigger enough to fire another round. This way you dont have to release the trigger completely. the trigger pull shortens and follow up shots can occur quicker. Old m&ps didn't have this. The shield does. Many M&P owners installed a part from APEX to introduce this click, but it appears that S&W listened to the consumer and added this feature.

__________________
It's harder if you're stupid