Just to add to the very thorough response of @"Accounts Playable", "Or" just precludes the world where both J and T are both out, since you must have one or the other or both. This was very hard for me to understand initially and reviewing the core curriculum lesson on "or" and "not both" really helped me make my way towards understanding.
Comments
Not J-->T
What can happen?
J is in. J is out. T is in. T is out.
If J is in, the rule is irrelevant. T can float in or out, which makes this "or" statement inclusive since it's OK for T to still be in.
If J is out, we know T must be in (since this is an "or" after all).
If T is in. The rule is irrelevant. J can float in or out; again, making this option for J an inclusive "or."
Finally, if T is out, J must be in.