Voting Api + Vote Up / Down = Vote Power
The vote will be casted by calling the Voting api function votingapi_set_votes(). But if you want to change the votes before sending it to Voting api there is hook for this called hook_vud_votes(&amp;$votes) which you can use.

Update:
The api file mentions a hook called hook_vud_votes(&amp;$votes) to alter the vote but in their code they are invoking the hook using drupal_alter(), so the right hook to implement is hook_vud_votes_alter(&amp;$votes), I think it is mistake in their api doc.