This trick called a "howlin' squeel." Its very easy to do, and it involves natural harmonics. All you do is hit a N.H.(ex.1) and dive. Does'nt sound to bad, but when you throw 2 of them into the mix(ex.2), it sounds EVIL, lol. If you pull up before pushing down (ex.3), it tweaks the sound even more. Also, adding full reverb and diving a little slower will make it HOWL!!!
This next one is a climb. Again it involves N.H.,but it rises. Just depress the bar all the way down, hit the harmonic and let it back up . Again add reverb to make the note sustain when it reaches its peak.
ex.4 / ex.5 (+5) <-----climb to pitch 5 steps up from
-5 * +5* -2 +5
H.
|--\--------/-| |---------------------|
|---\-----/---| |---------------------|
|----\3/------| |---------------------|
|-------------| |-------------/--2----|
|-------------| |--\-------/-----0----|
|-------------| |---\-0-/-------------|