Keeping your pinkie curved on your bow is important because it gives flexibility on your bow hold. It is essential to have your pinkie curved for smooth bow direction changes, good spiccato, and many other techniques.
If you are having a hard time keeping your pinkie curved, consider these simple things we will talk about here.
Bring the middle two fingers deep
The first thing to do for a curved pinkie is to keep the middle two fingers deep on the bow. If the contact points of these fingers to the bow are just the tip of the fingers, your pinkie has to be pretty much straight for it to stay on the bow. Bring the middle and ring finger, especially the middle finger, deep on the bow. Then all of your fingers will be closer to the bow, and that should help all the fingers stay curved.
Bending and straightening of the joints
In order to keep your bow in your hand, you have to bend either the 2nd or the 3rd joints from the top of your fingers (I am sure there are appropriate medical terms for these joints, but I will not get into that). You cannot hold the bow if all of the joints are straight or bent. To understand this, we will perform the following experiments.
I will explain it with a pencil instead of a bow. A pencil is lighter than a bow, and it is easier to practice with a pencil at first. You can move onto practicing with a bow once you understand what to do with a pencil.
1. Place your bow in between the 1st and 2nd joint of your middle finger
Place a bow on the fleshy part of your middle finger, in between the 1st and 2nd joints. Then, lay the bow across your fingers. We are trying to avoid holding the bow with the fingertips. This will ensure that your bow will be held deep in your hand.
2. Keep the 2nd joints straight, 3rd joints bent
Let's try holding your bow with the 2nd joints straight while the 3rd joints (these are the ones at the base of your fingers) bent. You will see that you can hold the bow with your fingers straight, including the pinkie. This is what we want to avoid, this is a bad example of how to hold the bow.
3. Bend the 2nd joints, keep 3rd joints straight
We will do the opposite now. Hold the bow with the 2nd joints bent while the 3rd joints straight. You will see that you can hold the bow with your fingers bent. This is what we want to do when we hold the bow.
Now you know what to do to keep your pinkie curved on the bow. When the 3ed joints are bent, your fingers will straighten. If you find yourself holding the bow with straight pinkie, try keeping the 3rd joints flat. You will see the improved result immediately.
There are a few other things we need to consider besides what we already talked about.
1. Keep your hand relaxed
Don't get stiff! You don't have to grip hard to keep your bow in your hand. Loosen up as much as you can, and you will find it easier to bend your pinkie, too.
2. Tilt your hand towards the index finger.
By tilting your hand towards the first finger, you will feel more weight from your arm on your index finger. You don't want to really feel much weight on your pinkie unless you are at the very frog and need to balance the bow. You will find it easier to keep your pinkie curved when it feels no weight from your arm.
We will talk about these in depth in How to relax your pinkie on the bow.