For decades, hitters have been taught batting mechanics based on the "Truism" that energy for the swing came from the linear transfer of their weight. To accomplish this, they were told that they must rotate their backside around a "posted" lead-side. If they allowed their lead-side to turn open as the backside rotated forward, it would result in "spinning" which would greatly reduce the power for the swing.
This "Truism" just explained has been supported by nearly every batting authority, exception being Ted Williams, the lone dissenter. Jim Lefebvre's book, "The Making of A Hitter" is one of the most quoted sources of this theory. Below are excerpts from his book.
"The back side of the hitter's body, including the rear hip and shoulder joints, rotate forward in the approach phase. Against this motion, the hitter's front side is firmly maintained. The front side of the body remains closed until forced to turn and open by the back side "clearing out" the front side. The hitter should think of this motion as part of the transfer of weight from the back foot to the front foot, not a simple swiveling action."
"These procedures permit a free flow of forward-directed centrifugal energy to power the swing". - "Hitting power cannot be produced by a simple turning or spinning motion of the entire body. In fact, such a motion can produce undesirable, inward-directed centripetal force."
Today, those that support the above theory, use the analogy of "a gate swinging on hinges." The lead-hip and shoulder joints act as hinges for the backside to swing around. This Web Site (and Ted Williams) has always maintained that although there may be a transfer of weight during that stride phase, the power for the swing actually comes from the rotation of the body around the center of the body -- the spine.
This means that as the backside rotates forward, the lead-side is evenly rotating rearward like a "revolving door." -- For those of you that might say the hips may cock (or slide) forward during rotation, let me point out the following.
(1) During rotation, most great hitter's heads remain almost stationary. Therefore, their shoulders are rotating evenly about their neck (spine).
(2) The bat/hands/wrists/arms segment is attached to the shoulders -- not the hips. Therefore, the hand-path and bat trajectories are governed by the axis of rotation of the shoulders -- not the axis of hip rotation as the clip below shows.
Giambi momentum & lead leg