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@ianjq Dezaki and his partner Sugino were masters at handling limited animation. Among the various techniques they used, the post-card memories are probably the most emblematic of his style. If you haven't, please try to give a watch to Dear Brother. Is available on Retro Crush.
My take for the shadowy faces is that they symbolize an emotional detachment. They're used for the moment a character has embraced the worst within them. They feel like voids ready to drag the innocent ones if they're not careful.