I have never had anyone show me how to do it before, so I need to know if the sound you get is the result of the string smacking the fretboard, or should the sound come from just hitting the string hard with your thumb and bouncing off. In other word does that metallic sound come from hitting the string against the frets or am I doing it all wrong?

Ever slapped on a fretless? That should clear up your question. The trebleish sound comes from the string hitting the metallic frets. You can get a slap sound on a fretless, but it's without the highs because the wood is softer.

That clears it up for me, Ive been doing it right. THNX