In theory it isn't difficult: when we see someone walking, it isn't only their lower body that moves. Their upper torso moves as well. For example, when the left leg moves, the left arm will move with it.
理論上並不複雜，我們注意人走路時，不只下半身會動，上半身也會動，譬如提左腳、左手臂也會動，就可以把上半身的訊號送下來，也許走得不是很漂亮，但確實可以走。