Very good question, Mike.
Open passing means that you can pass in turns and on straights. Point-bys are strongly recommended, but obviously this cannot always safely be done in the middle of a turn, so it is not required. The rule of thumb, at least at the advanced level, is to have cars pass you on the side that your next turn will be. So if your next turn is to the right, you should stay on your line, but let the passer take his or her pass on the right. If your next turn is to the left, the passer should pass on your left. This is because you have a better view of where the driver is than he might, and you can help keep both of you safe during the transaction. Occasionally, being as the Subarus and say, Miatas, are of similar horsepower, you may have to lift a little to let the transaction take place safely. If that does not make sense, PLEASE post such. I have a lot of track time and racing experience, so I've learned how passing should be done -- I just don't always word it perfectly.
Questions like this one are awesome -- I want to make sure you all are safe at all times!
BW