There are more theories of hitting a baseball than there are people who have ever hit a baseball.
Recheck the Bonds vid above, his motion to the ball isn't upward, the movement of his bat to the point of contact is downward. It is a misnomer that any good hitter's normal swing is on an upward plane getting to the ball. The only way to physicall swing "up" on a ball is to swing at a ball in your eyes. No one advocates a swing that continues downward after contact with a baseball (where the batter's hands and swing would finish below his waist)...that's ridiculous. As is the idea that there's an upward swing on a ball for any successful hitter.
If a baseball bat was flat (as in cricket) an upward trajectory of the bat would be needed to "lift" the ball...the fact that a baseball bat is round eliminates the need to have an upward titled plane of a swing in order to "lift" a baseball.
A baseball "swing", like that of BB's is a "backsided swing"...with little to no shift in the center of gravity (weight stays back). At the point of contact, the back knee, back hip, back shoulder and head are all in a vertical line or slightly titled back. Bond's is "short to the ball and long through the ball" as it should be. In a "swing" such as BBs the force is generated with the largest muscles in the body (in the back leg) exploding the back hip (torque) into a closed "front side" which creates a ratcheting effect of the force up the body (this is why core strength is important...to efficiently transfer this force) to the shoulders and bringing the back side of the body to the point of contact and out through the back arm as it explodes through the zone.
However, the "mechanics" of hitting a baseball pale in importance when compared to the skill of "watching the baseball"...this is the fundamental (and most ignored) skill in hitting. There are no "good swings" at the wrong pitch and there is no differentiation of the right and wrong pitch without the gathering of sufficient visual data. There are substantial and identifiable differences between the "gaze control" (or how the ball is seen) between good hitters (those who actually hit the baseball) and batters (those who stand in the batter's box and try to hit a ball).