Hello @steamsteem,
First, I want to thank you for your support. Really appreciated!
Then the answer to your questions:
Your vote for a witness is valued with your Steem Power. You are "giving" the same amount of Steem Power to each witness you vote for.
If your Steem Power change (Power Up or Power Down), the weight of your vote change automatically.
Feel free to contact me if you have any question.
Take care!