Normally, you pull off from a higher note to a lower note. Also, a pulloff is confined to one string. In essence you sound a higher note and then by pulling your finger off the string, you let a lower note sound, either by having an open string , or a fretted note at a lower position on the fret board. Press the string at the higher note, pluck the string and then pull your finger off the higher note and let the lower note sound. it takes a little practice to get that lower note to sound decent, but it shouldn't take too long.