We have been a dominantly right-handed tribe for millenia, so even though I don't know why a paleolithic person would trace his hand or blow pigment on it from a tube, I can easily picture how he did it.
何千年にわたって 右利き優勢の種だったわけです だから 昔の人が手形をトレースしたり 絵の具を吹き付けたりした ー “理由”はわかりませんが “方法”は簡単に想像出来ますね