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Japanese teams hire retired or washed up pitchers that are there to pitch simulated game-style pitches to batters for practice.
Usually there's one LH and one RH and they're used to polish and practice against like-handed pitching.
Extreme platoon splits are fairly uncommon in Japan and it's also why you see very little of it in the elite hitters that make it to the MLB.
You might see a power dip, but for the most part, it's things like BA in exchange for PX.