The term “Alternate String Pull-Off” (ASPO) was, as far I can tell, coined by Ken Perlman and received wide audience* with his publication of Clawhammer Style Banjo (an excellent book, BTW, one which I highly recommend).
I, though, do not use the term “ASPO” in my teaching, and I shall now take a moment to explain just why that is!
First of all, let’s define our basic terminology: a “pull-off” is a common, colloquial term for what the classical world calls “left-hand pizzicato,” i.e. plucking an instrument’s string with the noting hand. Thus, for right-handed players, we’re speaking of picking a string with the left hand.
Typically, one does a pull-off by fretting a string, plucking it with the right hand, and then plucking it again by pulling the left hand’s fretting finger to the side, sounding the open string (or a lower fretted note on that string).
This video shows a pull-off done on the banjo’s first string: first I fret the string at the 2nd fret and strike it with the right hand. Then, I snap my finger off that string, sounding the 1st string open (the tablature is below the video):
This is where that “alternate string” term comes from, i.e., instead of pulling-off the string plucked by the right hand, one pulls-off a string that was not struck by the right hand. In this video, I am sounding the 3rd string open, and then pulling-off the 1st string from the 2nd fret. Here, too, tab follows:
As the 1st string is pulled to sound the open string, it follows that it does not matter from which fret the string is pulled–you’re going to hear the open string! In each of those above videos, though, you’ll notice that my left hand is making the same motions, as I am pulling-off from the same fret.
In fact, I made this following video specifically to make this point in a post to the Banjo Hangout some years back:
And so this is why, in my teaching, I do not differentiate between “ASPOs” and “pull-offs,” as they are exactly the same thing–the only difference is which string the right hand hits, and that has nothing to do with pulling-off!
* Or at least as wide an audience as any banjo book is likely to reach…