Watch the The Smack Technique online guitar lesson by Ben Lacy from Two-Hand Groove Guitar
Here we're going to use a 1-fret chord, like Am7 at the 5th fret. Now we slap right over the fret 12 frets up from that. You can use any finger. I prefer my middle one.
You need to be accurate when you hit the fret for it to sing out for you. If you're not right over the fret it won't sound right.
You're taking all of the notes up an octave and getting that harmonic. Your smack should make that chord ring out.