A knot identical to a "whip stitch knot" is used to tie the qiangin string. This knot is often used to wrap the end of a
rope to keep it from fraying. The knot is also very similiar to a "hang-mans" knot.
Here's a finished version:
As a side-comment... you can see the rack I made for hanging my Erhu collection in the background of the previous picture. This is a simple oak towel bar from the hardware store mounted to a pine 1/2" x 6" by 36" plank. The original towel bar was only 18" long, so I bought a new pine dowel to extend the bar to the full length of the plank. I then hung one of my nice Navajo rugs over the plank to cushion the Erhus so they don't rub against the wall. You hang the instruments by the tuning pegs. A large number of instruments can be hung together if you hang every other one by the top peg and then the bottom peg. This staggers the sound boxes so that you can double the number of Erhus that will hang side by side in the same width.
The loops should pull the string back until it is away from the stick by something like the width of your index finger's first knuckle. Since I have big hands (long fingers), I tie mine the width of my thumb's knuckle. Most Chinese players look at my instrument and tell me my strings are too far from the stick. However my teachers have suggested this adjustment to compensate for my long fingers. I'd suggest starting with a width equal to the first knuckle of the first finger, but making further adjustments if you feel too cramped.
You start by making a loop which runs down the stick for an inch or so and then comes back up to the top. You then start wrapping over the loop working your way down the stick. I do five wraps around the stick, then go around the two strings, a wrap around the stick, a wrap around the string, alternating for 4 times, and then the final wrap around the string is doubled. You then wrap 5 more times around just the stick, and put the remaining piece of string through the little bit of loop that is still uncovered. Then you pull the string at the top and it pulls the loop and the last bit of string inside the wrap. At which point you can cut the extra off and you have a very neat job. Here is a schematic of the wrap just prior to pulling the string at the top to trap the bottom "U" section and the loose end into the wrapping:
stick string | | | | | | | | | | | | < loose end from U shaped starting string | | | ----- | ----- | ----- | 5 wraps to start ----- | ----- | --------- < around string ----- | < only around stick --------- < around string ----- | < only around stick --------- < around string ----- | < only around stick --------- < around string ----- | < only around stick --------- < final two are together with no wrap around stick --------- ----- | ----- | ----- | 5 wraps to finish ----- | ----- | ----U-- | < bottom of "U" and end of string inserted into U | | | | | | | | |
I use a nice #10 or #12 white silk cord that I buy from a local bead making shop.
Here's an image from the Boyscout handbook showing how to make a binding whipstitch to keep the end of a rope from fraying. It's the identical knot used for the qianjin except for the loops around the string. The only difference is that I usually tie it with the U part down rather than up...