Of course, learning anything new on the guitar is hard, but this is baffling me. 

I'm doing Anders' tutorial on this back beat slapping, and I really want to master the technique because the practice tune is beautiful.  Anders is very skillful and musically intuitive about selecting lovely chord progressions that are simple to understand and actually pretty easy to execute. 

It's the back beat slap that has me stymied.  I see Anders doing it and it sounds percussive.  I don't get any sound from the slap.  Do I need to slap harder, come at the strings at a certain angle, or use more fingernails?  What is the secret?  

FYI, I usually do these exercises at .9 or .75 speed so I learn to do them correctly rather than at tempo.  Anders warns against rushing and developing bad habits.  Practice makes permanent (not necessarily perfect) is a rule I force myself to remember.

Suggetions?