This seems to be something that's simple enough to not need explaining, but I'm going to need some explaining because for some reason I just don't get it.

In the No Upstrokes lesson, you seem to be hammering-on and Pulling off the A and D strings with just your index finger. I can do them individually but when I try to do both I usually end up pulling through the G string as well or I pull off too slow and they ring out in sequence instead of as a chord.

Do I just need to be more careful and precise in this motion or is there something else to it?