[Yoda]

The book references both accumulator #4 (9th component) and accumulator #1 (12th component). Why did Homer reference both of these accumulators in 12-5-1 if we are only using one?

Either the Left Arm (Pull / Swinging) or the Right Arm (Push / Hitting) is the Active Direct Drive of the Basic Motion Stroke. The other is Passive and supplies merely motion. But, no matter which supplies the Active Direct Drive, the Left Arm swings and the Right Elbow bends.

The Active Direct Drive is the player's choice per the Basic Patterns of 12-1-0 (Hitting) and 12-2-0 (Swinging). A bystander cannot tell which Force (Pull or Push) is being employed. Study 10-11-0-1 and 10-11-0-4.