This short video explains the unique rear sight of the early Japanese Type 99 rifle. These were setup so that troops could fire at passing planes.
More about the Type 99 can be found at C&Rsenal:
C&Rsenal releases their flagship series “Primer” every other week. It focuses on one firearm of the Great War at a time, in depth with animations, live fire demonstrations, and historical context! Our mission is to document and describe historical military small arms from across the world. We hope to share our love for all the attention that went into the design, development, manufacture, and issuance of these pieces.