OKAY! I hope you're ready to put some time and pratice to learn this. You will need a lot of patience and precision... I don't think this is a 'real technique' but I have decided to develop it. I want to mention that I'm not considering myself as the inventor. Les Claypool and Flea was using the "Double-Slap" in some tunes before me. It's a song of Primus that give me the taste to practice this. Let's start with something boring. This is for praticing the FUNDAMENTAL alternance 'Thumb-Pinky'. For a beat, you should hit the string 'Thumb-Pinky-Thumb-Pinky' (as 1/16 notes) just like if you were doubling thumped 1/8 notes. For more precision, you should place your hand perpandiculary to the strings and exagerate the movement. I hope it's not hurting your fragile little pinky finger! I think this is more difficult to pratice slower than faster. So, I choose a intermediate beat of 120. Feel free to raise it if you have too much difficulty but, try to be comfortable at regular speeds, this will help you. It's normal to miss some notes sometimes, I have still difficulty myself to hit them all with precision... Now, PRACTICE!!!