The ranking is a function of votes, flags, and time. Not all votes are equal, as some votes are determined to be correlated via voting-ring type considerations.
But if a submission gets four or five independent votes very shortly after submission then it can easily make the Front Page. It won't happen if the votes trigger the voting-ring detector, or if the submission is flagged by someone.
Why do you ask? Why are you surprised? What other reading have you done before asking the question?
I was thinking the other day: asking a few friends to up-vote submissions is common practice and something probably done all the time on HN. On the other hand, deliberately creating new accounts to up-vote all your sockpuppet account submissions can be detected. HN doesn't go into how they detect it for obvious reasons though.
2. dang emails people who submitted items that were good but didn't take off and then puts them into a system that pushes them to the front page at a certain time to see if they would work once there. This has happened to me a couple of times.