The violin bow hold is something of an art. Relaxed hand. Curved thumb. Bent pinky. Arranging the hand can be a gymnastic challenge to master.
My violin educational resources include YouTube videos made by people who seems to know what they are talking about. RedDesertViolin and ProfessorV are two that I have learned a lot from so far.
The geography of where each finger is supposed to be on the bow is explained very well in RedDesertViolin's video. Not only is the instruction good, the finger positions are easy to see and understand. Making my fingers do it is another story though ! In the above picture, my index finger is too far forward, and the pinky is certainly not bent. More to improve on !!
Violin practice sessions give me a number of things to work on. I have to learn how to hold the violin and how to get my left hand fingers positioned to play notes. I also have to focus on musical elements such as tempo, volume, and how to phrase notes with the bow. Plus, I need to strengthen the muscles needed to accomplish it all. After initial soreness, it is satisfying to see the muscles improving and helping me move as I need to.
ProfessorV's version of the same bow hold instructions reinforces what I need to learn. His voice is soothing and gives me hope that my bow hand will soon be doing the correct grasp !
(The bow string is made of horse hair, and the black part at the base of the bow is called the frog)