The simplest bowing is the pull stroke (bow going to the right) on the front string. The stick should *not* be held up, but should rest gently on the body of the Erhu. Let the stick fall forward and slide on the plastic bow-guard. The intent should be on the point of contact between the hair and the string with a slight forward and down feeling.
In the following picture, notice the following points:
The bow is held or controlled by these two things: the grip of thumb and finger and also the pushing of the thumb against the joint of the finger. The degree of control varies between these two points at different stages of the bowing. If you only had one way to grip the bow, your control would be rigid. With two ways to grip the bow, you can easily adjust either point of control to shift confortably as needed.
This control will be used for playing the back string. For now, just let the fingers rest and be relaxed.
I like to tell people that holding the bow is like riding a horse. The thumb holds the saddle horn, and the middle and ring fingers sit in the stirrups. Front string is played "sitting down", and back string is played by pulling back on the middle finger "standing up" in the saddle.
Since this is a "pull bow" the wrist is to the right of the thumb. The wrist leads the fingers, the forearm leads the wrist, and the elbow leads the forearm. The forearm is lead by the shoulder and the shoulder should be lead by the breath.
This is a push bow (bow going to the left). Now the wrist is to the left of the thumb. Continue to think/feel "down and forward". The attention is focussed on the point of contact between the hair and string.
Now we are pulling on the back string. The middle and ring fingers are pulling the hair back by "standing up" in the "saddle" The pulling is against the point of control where the thumb pushes against the index finger. The hair should now contact the back string rather than thr front string. The feeling is straight back. Try to make the main pulling motion dominantly at the fingers. Don't get in the habit of pulling the bow back with your arm, wrist or shoulders. In all cases, the arm and body must be relaxed. If you don't take care here, you will have difficulty to rapidly alternate between the strings. The switching must be done mostly with just a smooth adjustment of the fingers.
The wrist angle varies over the whole range of bow. When you first start pulling a long bow, the wrist angle is steep:
Near the end of the pull-bow, the wrist angle is lessening, in preparation for the push-bow.
At the very end of the pull, even while you are still pulling, the wrist continues to change angle until are in the pushing position. Only after the wrist has finished changing angle does the bow direction change. This staggered changing is very important to get a smooth sound.