It seems helpful to consider an extremely simple scenario. Suppose an astronaut is floating near two balls of lead. She can pull the balls together without changing the momentum or angular momentum of the system. She can then rotate them in the center with almost no change, and separate them again. If that little twist in the center bothers you, you can imagine that she instead has three lead bars, pulls them together so that one slips between the other two, and then pulls them apart along a different axis. The actual mechanics of human and cat motion are more complex, of course, but you can think of movements like raising and lowering the arms and swinging them forwards and backwards as essentially similar.

Imagine that your whole body is held rigid and straight except that you can swing your arms at your shoulders. Start with your arms at your sides. Now lift them up and forwards as though you were bumping a ball in volleyball, until they are perpendicular to your body. Your body will tilt forwards. Now pull your arms apart, to the left and right. You will again tilt forward. Finally, push your arms back down to your sides. You will not tilt at all, but will return to your original body shape, only tilted forwards relative to your original orientation.

It seems helpful to consider an extremely simple scenario. Suppose an astronaut is floating near two balls of lead; in this case the closed system consists of the astronaut together with the balls. She can pull the balls together without changing the momentum or angular momentum of the system. She can then rotate them in the center with almost no change, and separate them again. If that little twist in the center bothers you, you can imagine that she instead has three lead bars, pulls them together so that one slips between the other two, and then pulls them apart along a different axis. The actual mechanics of human and cat motion are more complex, of course, but you can think of movements like raising and lowering the arms and swinging them forwards and backwards as essentially similar.

Imagine that your whole body is held rigid and straight except that you can swing your arms at your shoulders. Start with your arms at your sides. Now lift them up and forwards as though you were bumping a ball in volleyball, until they are perpendicular to your body. Your body will tilt forwards. Now pull your arms apart, to the left and right. You will again tilt forward. Finally, push your arms back down to your sides. You will not tilt at all, but will return to your original body shape, only tilted forwards relative to your original orientation.

It seems helpful to consider an extremely simple scenario. Suppose an astronaut is floating near two balls of lead. She can pull the balls together without changing the momentum or angular momentum of the system. She can then rotate them in the center with almost no change, and separate them again. If that little twist in the center bothers you, you can imagine that she instead has three lead bars, pulls them together so that one slips between the other two, and then pulls them apart along a different axis. The actual mechanics of human and cat motion are more complex, of course, but you can think of movements like raising and lowering the arms and swinging them forwards and backwards as essentially similar.

It seems helpful to consider an extremely simple scenario. Suppose an astronaut is floating near two balls of lead. She can pull the balls together without changing the momentum or angular momentum of the system. She can then rotate them in the center with almost no change, and separate them again. If that little twist in the center bothers you, you can imagine that she instead has three lead bars, pulls them together so that one slips between the other two, and then pulls them apart along a different axis. The actual mechanics of human and cat motion are more complex, of course, but you can think of movements like raising and lowering the arms and swinging them forwards and backwards as essentially similar.

Imagine that your whole body is held rigid and straight except that you can swing your arms at your shoulders. Start with your arms at your sides. Now lift them up and forwards as though you were bumping a ball in volleyball, until they are perpendicular to your body. Your body will tilt forwards. Now pull your arms apart, to the left and right. You will again tilt forward. Finally, push your arms back down to your sides. You will not tilt at all, but will return to your original body shape, only tilted forwards relative to your original orientation.