An affirmative minor tells us that the middle has instances falling under the minor term. The major can then describe these same middle-term instances with respect to the major predicate.
A negative minor excludes the minor predicate from the middle and does not locate any minor-term instances to which the major can be applied.
Example:
No horse is a bird.
All horses are animals.
Neither “Some birds are animals” nor “Some birds are not animals” follows from these premises. They describe horses, not the required bird instances.