By Bass Levite
I'm still learning this awesome skill myself, but I thought I could share what I know so that we can all learn together. Obviously the first step is to know your scales and arpeggios very well. This is the foundation, because most riffs are built on this.
Okay, what's a slap? It's hitting the strings with the bony part of your thumb, as you can see above. You need to release the thumb after striking the string, to allow the string to ring out the characteristic "metallic" sound. The best place to strike the string is between the pickups and the last fret. Practise until you can create a decent sound, and also be able to hit the strings accurately without accidentally hitting more than one string at once.
The second skill is the "pop". This is where you pull the strings from beneath with your index finger and then releasing it. The string hammers against the frets, creating a "popping" sound. Don't pull too hard! Just enough for it the strings to snap back at the frets when you release it.
Slap and Pop
As you can see in the video, my right hand is positioned such that I can alternate slap and pop by just rotating my hand at the wrist. This is the most common octave position, where the thumb slaps the E and A strings, while the index finger pops the D and G strings. The first round are long notes, where you let the string ring out after playing them, while the second round are short notes, where you mute the strings after playing them.
In the third round, I mute the strings with my left hand, and then slap and pop them with my right hand. This creates a percussive sound, called ghost notes (because no notes are being played). Ghost notes, like interval rests, are important to allow the riffs to "breathe". Playing notes all the time without rests or ghost notes makes it sound too aggresive and restless. You need rests and ghost notes in your riffs, so that the notes you play stand out. It's like black ink (notes) on white paper (rests).
Putting it all together
A typical bass slapping funk and solo will involve putting together slaps, pops, rests and ghost notes in various combinations. As for the left hand, you can do slides, hammer-ons, pull-offs, vibrato or just a simple note. Put the right and left hand techniques together, and you can come up with endless different combinations of riffs and solos.
However, these are only the technical bits. Like I said before, you need a firm grasp of scales, music theory and know your fretboard very well to be able to make up your own stuff. This is the "brain" bit. The "heart" bit is where you inject emotion into your playing, be it slapping or not. I'm sure some of you have seen an awesome musician play with great skill, but there's no emotion in it, making it dull. Similar for slapping. In fact, probably half of a good performance is made up of the "heart" content, while the other half is the skills bit.
The remix introduction of "Soldier" (http://www.youtube.com/watch?v=nHwTZQLmVZQ) is a relatively simple riff to learn and practise.
Use the octave position mentioned above, ie, the thumb slaps and E and A strings, while the index finger pops and D and G strings.
* indicates a ghost note (can mute the string anywhere)
~ indicates vibrato
h indicates hammer-on
Slapping in Church
Beware! The slapping and popping technique is very flashy, so use wih discretion. Personally for me, I'd rather not slap when performing solos and instrumentals in a church setting because it sounds very aggresive. Instead, salvation-is-here-styled riffs are more suitable. The exception to this rule are gospel songs, like some of Israel Houghton's songs. However, if during praise and worship, you feel that slapping would build up the atmosphere, then go ahead and proceed with caution.