You don't have to wait until expiry. Transfers generally add 1 year of service (for most TLDs), so unless you're already at the limit, you can transfer everything now.
It is mentioned in the first paragraph of the README:
shpool is a service that enables session persistence by allowing the creation of named shell sessions owned by shpool so that the session is not lost if the connection drops. shpool can be thought of as a lighter weight alternative to tmux or GNU screen.