Yeah. There is an announcement that it's about to happen, and when you sign up, pairings are later announced, usually with the intended theme also announced.
It is open to all members that haven't stiffed their PITH partner. You do your best and stretch your abilities to make the very best pen you can and sent it to your partner. You learn a whole lot in the process and get to receive someone else's effort. Great fun. My first one caused me many headaches and exploded blanks. Great fun and I learned a tremendous amount about segmenting. This forum is the single greatest place to learn anywhere. Join in on the next one. You will be amazed.
Charles