So basically each switch chooses between 1 or 2 diodes on each side, and it ultimately gives you three options:
- 1 on the left and 1 on the right is symmetrical clipping, with lower clipping threshold.
- 1 on one side and 2 on the other (doesn’t really matter which side is which, they’ll sound the same) is asymmetrical, one side will clip harder than the other.
- 2 on the left and 2 on the right is symmetrical clipping, with higher clipping threshold.
Between the two switches, you’ve got three possible sounds (and it would be a better design choice imho to use a single three-position switch than two SPSTs, since two combinations are equivalent).
@BuddytheReow ’s explanation of the tonal differences covers the tone very well