Bends can be heard in the bass and guitar parts of many songs. Bending notes adds a nice melodic flourish to licks and solos. On the bass, a string bend is often used to embellish a bass line with a dirty, bluesy touch on certain notes. Bending notes is simple, and only takes a minute or two to learn.
What is a Bend?
A bend is the technique of fretting a note and then pushing the string laterally across the fretboard (parallel to the frets) to increase its tension and thus bend the pitch up. You can push the string up or down. Either way, the pitch goes up above the pitch that you would normally hear at that fret.
To execute a bend, first play a note. You should use your second or third finger to fret the note, and put down your other (lower) fingers along with it. With two or three fingers down, you'll have more force at your disposal to bend the string.
Depending on which string you are bending, you'll want to either push it up towards towards the ceiling or pull it down towards the floor. The first string should be pushed up and the fourth string pulled down, to avoid having the string slide over the edge of the fretboard. With the middle strings, do what you find easiest.
Good fretting hand technique demands that you keep your thumb behind the fretboard, but when you are bending notes, you can break this rule. Some bass players still don't, but many find it easier to get good leverage for a bend when they let their thumbs hang over the top of the fretboard.
You can bend the pitch a lot, or you can bend it a little. If you bend it just a little bit back and forth, you can get a nice vibrato sound. You can bend it a lot as well, until you get the same pitch as you would if you played the next fret (a interval of a "half-step," or "semitone"). If you bend it about half this amount (a "quarter tone"), it's called a "smear."
Sometimes you will see bass tab that indicates you should bend the pitch up a semitone. This is about as far as you can bend the pitch of a bass string without significant effort. Guitarists commonly bend their strings to raise the pitch two or more semitones, but bass strings are longer, thicker and stronger, and can't go as far.