Watch the OSPO & ASPO online guitar lesson by Matthieu Brandt from Modern Mountain Banjo: Clawhammer & Up-Picking
In a clawhammer "bum ditty" groove, it's hard to get certain melody notes. The up and down motion of your right arm and hand makes it difficult to play certain combinations of notes without losing the groove, and the groove is the most important part.
This means that we have to resort to some trickery if we want to introduce these combinations of notes. One of them is the open string pull off. You lightly touch an open string with one of your left hand fingers and make sure you don't press it down on the fretboard or let it ring out. You then pull it off in a quick motion, towards the ground.
Another technique we can use is similar, but more complex. It's the alternate string pull-off. You already have a fretted note on a certain string. That means one of you left hand fingers is already occupied. With an additional finger, you lightly touch that same string and play a pull-off. These techniques are used a lot by modern players such as Adam Hurt. Listen to his rendition of "Rebel Raid" for a great example of open string pull-offs.