hang by (something)
To hang or dangle from something with something else (named after "by") serving as the only means of connection. You better go get your toolbox because that board is hanging by a nail. Look Mom, I can hang by one hand on the monkey bars!
hang by something
1. to be suspended at the end of something, such as a rope, chain, string, etc. The bag is only hanging by a string. The food hung by a rope from a tree to protect it from the bears that wandered into camp now and then.
2. to dangle, suspended by some body part, such as thumbs, legs, etc. I was hanging by my legs on the exercise bar when the rain started. I can hang by just my middle fingers!