This is definitely an important type of rule to understand: not J -> S means that you can’t have both J and S out, because when J is out then S is in, and when S is out then J is in. However, just because you can’t have both J and S out, this doesn’t preclude J and S both being IN. Given the rule and its contrapositive, if J is in then S could be either in or out. Likewise, if S is in then J could be either in or out. So the only possibility that the rule excludes is the possibility of both J and S being out.
A similarly important type of rule to understand: J-> not S means that you can’t have both J and S in, because when J is in then S is out, and (from the contrapositive) when S is in then J is out. However, just because you can’t have both J and S in, this doesn’t preclude J and S both being OUT. Given the rule and its contrapositive, if J is out then S could be either in or out. Likewise, if S is out then J could be either in or out. So the only possibility that this rule excludes is the possibility of both J and S being in.
Hope that helps 🙂