I wouldn't recommend jumping between different branches too often when you are deploying, it is too easy to forget which branch you actually want to deploy and that could be disastrous, but on occasion it just makes sense.
The command you need to run would be something like this;
ey deploy -e demo -r my_awesome_branch
ey just calls engine yard, deploy tells engine yard that we want to deploy something, -e tells it what environment to deploy it to, and -r is optional and stands for ref, this tells engine yard which branch, tag or SHA to deploy.