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1
Right vs Left
Tori faces uke in a typical right versus left situation. From here, tori could do either a right-sided morote-seoi-nage or a left-sided ippon-seoi-nage. The reverse seoi-nage works because uke has no clue which direction tori is going to throw.
2
Rotation to the left
Here you can see tori has started to rotate to his left. This would be a typical movement for morote-seoi-nage. Uke doesn't know it yet but tori is not going to throw him to the front. Tori's rotation will throw him in an unexpected direction.
The Rotation (Upper Body View)
REVERSE
SEOI-NAGE