When you take the pick and slide it down the strings.
Like the beginning of this tune:

http://s93744050.onlinehome.us/mp3/MoogBoog.mp3

Usually you take the edge of the pick and scrape it on the 6th & 5th strings all the way from the bridge and down the neck.
If you want to expand on it, you can slide down with the pick and then continue the slide down with your left hand while your right hand returns back to picking position at the bridge.