Just made a short video on how to rosin the Erhu bow.
The first part is on how to rosin a new bow. When I first started learning the violin, I gave my new bow a few strokes of the rosin and expected it to sing! Rosining a new bow is hard work, especially the Erhu bow because it is longer than the violin bow and you have 2 sides to rosin.
The second part is how I usually rosin the bow attached when to the Erhu. I like to put the Erhu lying down on my lap. To prevent the bow from moving about while you rosin, you might want to rest the tip of the bow on the snakeskin protector of the resonator in a diagonal position like in the video.