My understanding is fuzzy, but the sense I get is that the word "when" just attaches to the possible configurations rather than being a condition. A rephrasing of the idea might be:
"Multiply by 1 for the configuration in which the photon goes straight, and multiply by i for the configuration in which the photon turns at a right angle."
Which results in two configurations every time, rather than different configurations depending on what the photon does.
My understanding is fuzzy, but the sense I get is that the word "when" just attaches to the possible configurations rather than being a condition. A rephrasing of the idea might be:
"Multiply by 1 for the configuration in which the photon goes straight, and multiply by i for the configuration in which the photon turns at a right angle."
Which results in two configurations every time, rather than different configurations depending on what the photon does.