TRILL: This is just a fast hammer-on then pull-off played continuously. Generally trills are not rhythm based, if a song is slow of fast the trill is usually the same speed.
When playing a trill only pick once, the continuing sound is made by the rapid hammering and puling away of the fretting hand.
Even though this tequnique is not commonly used, it's still a good idea to practice it and be able to hear it being played in songs. If you're already good with hammering and pull-aways then trills are a natural progression.