It's assumed in TGM that the swing center is the left shoulder, except for the variation where the swing center is shifted to the right elbow. However, there are other alternatives, such as what Goosen does. He shifts the swing center to approximately the space between the clavicle joints, below the adam's apple. This is why his left arm is so severely bent, and does NOT require a flat left wrist through the impact interval. Note, that Goosen is still a swinger. Homer notes that the flat left wrist imperative is relaxed somewhat for hitters, but with the swing center moved away from the left shoulder, it can also be relaxed for swinging.