'down ' beats (when u would pick downwards with the pick), and use the third finger for the 'up' beats. but u dont have to do this. i like this technique because u have all the benifits of a plectrum, eg pinch harmonics, speed with runs, but can also play three notes simultaniusly without having to stum across the whole six strings, and string skip with relativly easily. also your fingers sound slightly pifferent to the pick so it gives you some tonal controll. and for all u speed freaks the more fingers u can use the less you have to move the pick the faster you can go.
a good song to practice this on is the intro to sweet child of mine.