The purpose of r53up is to bind a fixed DNS subdomain to an AWS EC2 instance which is started on-demand and does not have a fixed public IP address.
It has a simple command-line format: r53up hostname domain
e.g: r53up myhost mydomain.com will bind myhost.mydomain.com to EC2 instance which ran the command.
It is designed to be run automatically at startup and it has a short DNS time-to-live (60 seconds) so that the host's new IP address is available fairly quickly after an instance is started.