This rule highlights the importance of impairing the opponent's vision. By attacking the eyes or otherwise obstructing vision, an opponent becomes vulnerable and less able to defend themselves.
The third rule is perhaps the most brutal and the first time we see Daniel implementing the lessons in a real world environment as Silver arranges for a man in a club to attack Daniel leading him to punch the man in the face to block his vision. Daniel feels great remorse for this action but is encouraged by Terry who tells him he has done the right thing.
It is at this point that Daniel begins to realize that the training he has been receiving might not be entirely what it initially seemed.