They've completely revamped the procedure of picking externships since I graduated... so we probably need someone more recent.
When I was there, we picked by class rank. The top 25% of the class picked first and then they released what was left to everyone else. I'm not sure they still do that.
You are required to do one at home rotation at UHCO , and at least one path based rotation (called a B site). An A site is usually more primary care focused. You can do TWO B sites, or 1 A site and 1 B. You cannot do two As. Some are also marked A/B. You do a total of three rotations.
Everyone will give you their specifics on when they think is easiest to do home rotation. I did it Spring semester so I was home for Boards and graduation. But, they are harder on you Spring semeseter because they know you've been out learning for the last two semesters and should be able to function with little to no supervision.
Your home semester you are also required to do a Grand Rounds presentation over a patient you've seen, while in front of the students and faculty. It's not as scary as it looks. My tip here: GO FIRST !! They are WAY easier on people who voluteer to go the first week and prep in three weeks than the people who have all semseter to write and then don't know answers to questions when they stand up to present. Get together with the other people going your week and present in the lecture hall in front of them to get feedback, iron out issues, holes in your presentation, etc.
Hope this helps!