The First Engine is the lower gear seen above. The First Engine is the rear leg. High level hitters are one-legged, meaning the vast majority of their weight is in/over their rear leg AT LAUNCH. Their lead leg is on the ground. It supplies balance. But it has no significant weight on it at launch. It has no real function in the swing other than to 'catch' the swing so you don't fall over as a result of the momentum created. In the act of 'catching' the swing, the lead leg will provide a 'block' so that the energy created continues around the rear leg and into contact rather than bleeding forward linearly. But it does not actively push back. It blocks.

The rear leg is the driver of the swing, not the hips. The rear leg is loaded by the coil of the hip and the pulling back of the muscles of the back against the leg. A spiral-like load around the rear leg is created. Strong external rotation pressure will be felt by the rear foot at the ground. This pressure is caused by the coiling hip. It is not caused by any external rotation of the leg. In fact, this hip coiling action is countered by the leg's INTERNAL Rotation. A rag wring-like load is created in the rear hip socket. The hip is coiling rearward as the leg tries to turn forward. A bind is created. This means, the rear leg, while attempting to internally rotate actually has strong external rotation pressure applied to it because of the strength of the coiling hip. The rear hip coil is done by the rear hip socket coiling around the ball of the rear femur. The pelvis is a solid bone so the entire pelvis will pivot as this coiling occurs in the rear hip socket. However, the pivot point of the pelvis' turn is the rear hip socket. It is not the center of the pelvis which would happen if the hitter swung around a spinal axis.

As the hitter picks up his lead leg and starts to move out, his back muscles will begin to counter that forward momentum by pulling rearward in the same direction of the hip's coil. This action is shown well by this clip...



I call the above the 'Missing Frames'. Because they are missing in swings that are not high level. Most amateurs and many pros do not have the missing frames. But every great hitter has them.

Pedroia is using his back muscles to ARREST his forward momentum. That action creates a strong rubber band-like stretch around the leg. The rag wring-like stretch that was already established in his rear hip socket is enhanced by the muscles of the lower back and scap pulling rearward against his forward momentum. This action maintains the corner, around which the barrel is launched. Without this action the hitter would turn forward too soon and thereby bleed his stretch. Notice his center of gravity lowering. That happens as a result of the back working to arrest the momentum. The degree the center of gravity lower varies and it is mostly based on the hitters original stance and the amount of forward momentum he uses. But no matter, the hitters goal is to 'stay on the barstool'. Stay on his pedestal which is his rear leg. The hitter does not allow his weight to shift forward. The stretch is 'around' the rear leg and up the back to the scap. The front side has no involvement in this stretch process. Pedroia's lead leg extends forward involuntarily to maintain the body's balance. It moves much like your arms move when walking. You don't think about it. But they move on their own to provide balance. Front side involvement would require weight to be shifted forward. If weight is shifted there is no stretch around the rear leg. You can't have both. You can't have a quality rear hip/leg load and have weight forward. In order to get weight forward you MUST bleed any load you had in the rear leg.

When the above load is achieved, the hitter simply turns the barrel rearward and that triggers the leg to snap through the 'hold' of the hip and back and the barrel gets whipped forward with a quickness that no other technique can match.