This page only uses your private key to sign the transaction locally. Private keys are never transmitted. You can download all the code and run locally if you prefer. There are more secure ways to vote, such as cli_wallet.
How to Vote:
1. Enter your account name and private key in the fields below.
2. Click the check box for every delegate you want to vote for.
3. Each check box click is an individual vote.
4. To unvote, uncheck the checked box.
5. That's it! There is no 'submit button'.