If your bylaws state that you follow "Roberts Rules of Order", then you may not need to explicitly state this. The idea is that the chair may be able to bear undue influence so generally should remain neutral.
The rule is that the president can vote 1) if the vote is by ballot (since no one knows who voted what), or 2) when his/her vote would change the results.
The most common scenario of #2 is "to break a tie" thus we see that wording a lot.
The less common case would be to force a tie. Say the vote is 5 against, 6 for that XYZ will happen. If the president votes "against", it forces a tie. Since a majority is needed to carry the vote, the motion would fail. Had the president not voted, the motion would have passed. Other examples, might be if you were only 1 vote away from a 2/3 majority needed.
Note this rule about voting applies only to issues within the business meeting. Like anyone else, the President has the right to cast a vote in elections.
Here's a link that may help you:
www.constitution.org/rror/rror-10.htm#58
[ 05-07-2003: Message edited by: JHB ]</p>