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Ok the above image shows Tyson getting a strong boost of power from his legs and instead of channeling it into his hips rotation he channels it upwards in the direction of the uppercut. You can tell this by how his whole body also lurches up in the air and I believe his chin pops up too.
You can also channel the legs into the rotation. Giving extra rotation and extra power for the uppercut. Notice how georges head and shoulders rotate far more violently than Tysons, because all the force is going into rotation, instead of some into rotation some into upwards linear force.
I much prefer foreman's way, it feels more powerful, faster and theres no tendency to lift your chin up.
However most boxers from what I have seen do it like Tyson not like foreman, which I think makes for a pushier slower weaker punch.