I figured it would be useful to have a thread (perhaps even stickied) that covered the key mapping on the NES30 Pro. I imagine this is a pretty popular controller for the RetroPie, but I'm not sure if anyone has ever listed all the commands. Below are the commands I know, please feel free to add to the list.
Specifics about how I mapped mine...
Left Trigger - L1
Left Shoulder - L2
Right Trigger - R1
Right Shoulder - R2
To Save: Hold R1, then hit Select
To Load: Hold L1, then hit Select
To Change Save #: Hold Select, then push D pad left or right arrows (<>)
To Move in Slow Motion: Push R2 (push again to undo, enemy will still move at normal pace)
To Go Back to RetroPie: Push Select and Start at the same time.
A note about save slots, they are specific to each game so you don't have to worry about saving other games to the same number. Each game will default to save slot 0 when you start the game.